Who were these people? Considering Portugal's small population today (10 million), that seems like a huge number of colonists. Had they been there long? I know the portuguese had been exploring and colonizing Africa since the fifteenth century, does their presence date to that period or was it a more recent phenomenon?

It was recent phenomenon, from the 1930's onwards there was a huge increment of people moving from rural areas in Portugal to Africa.

What was their status in society? Were they largely bureaucrats? Merchants? Farmers? Was their anything resembling South African apartheid?

All ranks in society were hit by the the forced return. There were some racial divisions, but nothing like Apartheid, social movement of non-whites was allowed - and in some cases people from one province/colony would serve as colonial authorities in other province/colony - and a big chunk of the Portuguese Armed Forces was made up by locals (almost 50% by 1974).

Was it expected? Did the colonists see the "writing on the wall" and perhaps expect what was to come? Or were they blindsided by it?

I think most people knew that eventually most colonies would become independent, but the speed of the events that led to the independence triggered by the 1974 revolution took people by surprise.

Was it "necessary"? In other words, if the Portuguese people hadn't left, what would have happened to them? Would they have continued to live peacefully (albeit in perhaps reduced circumstances) or was the political situation such that they would have been forced to leave in any event?

Infighting between guerrilla factions and plots for a "white independence" in some places forced people to flee to Portugal.

Who were these people? Considering Portugal's small population today (10 million), that seems like a huge number of colonists. Had they been there long? I know the Portuguese had been exploring and colonizing Africa since the fifteenth century, does their presence date to that period or was it a more recent phenomenon? If they had been there many years, what was their relationship with Portugal like?

Portuguese colonization was different from most colonial enterprises of other European powers. Due to the small population and colonizer recruitment pool, most colonial ventures were "feitorias", which were basically heavily fortified trading posts with a handful of men ruled by a metropolitan commander or bureaucrat. This was the Portuguese model of colonization for its African (Ceuta, Guinea coast), Indian (the small colonies of Goa, Damão e Diu) and Asian possessions (Macau). In Brazil the model was different due to its vastness, economic potential and "scarce" native population, and the lack of metropolitan manpower was solved by replacing it with African slave labour.

[Source](Diffie, Bailey (1977). Foundations of the Portuguese Empire, 1415–1580. University of Minnesota Press.)

With the independence of Brazil in 1822, Portugal loses its main economic and colonial endeavour. This is the period where most European powers gear up for the "scramble for Africa" and try to position themselves more favourably to attain their interests and slavery is abolished during the next couple of decades. Portugal is no exception in its desires for Africa, and its political leaders are bound to make of Angola the "new Brazil" and reinvent its economic role, as well as establish their domain from coast to coast, from Angola to Mozambique. And this is where true colonization that goes beyond mere trading and border posts, truly begins in earnest. Before this period, there were a couple of historical moments (just as with the Marquis de Pombal government or the end of Portuguese trade monopoly) that saw Angola prosper and then decline with the fluctuation of the metropolitan economy, inevitably falling back to its main vector of slave trade.

Who were these people? As with many colonial enterprises.. they were indentured servants and criminals, deportees, among them the (in)famous "Zé do Telhado", a real historical character that is a mixture of Robin Hood and wild west cowboy, and people looking for a break in life coming from the poorest regions of the metropole coming to Angola in transport provided by the government. They were the intermediate layer of colonial society, with the aristocrats and the military on top, followed by the colonial administrative bureaucracy, and the local populace at the bottom, managed by the old “divide and rule” tactics (bakongos vs. ovimbundos vs. ambundus).

The colonization effort goes into overdrive when the Dutch (or Boers if you prefer) start to encroach in the south of the Angolan territory in the 1880’s, and when the British reveal their plans of linking Cape Town to Cairo. As with any Portuguese enterprise of the time, it was ill prepared and eventually failed…

What was their relationship with Portugal like? Most people coming to Angola (or other African Portuguese territories) during this period would later seek to return to the metropolis, either by having served their sentence or completed their military service. By the turn of the century, the European population in Angola doesn’t even reach 10k people. The numbers only really kick-off during the period of transition of the military dictatorship to fascism in the metropolis. [Source](Kivouvou, Prosper, 1980: Angola – vom Königreich Kongo zur Volksrepublik, Köln.)

Was there anything resembling South African apartheid? Well… this is a much politicized subject, but I’ll try to give you my most objective perspective that was taken mainly from my personal interactions with “retornados” (those who “returned” to Portugal after the independence) and some conferences and talks that I the chance of attending, one with a famous journalist called Diana Andringa on the subject. The short answer is no. But there is a but…. It’s a colonial project, hence imperialism and racism are at its core. Subjugation of other people and other cultures are a sine qua non condition for the constitution of an empire. Portugal was no exception, although people think of Portuguese colonialism as more “benign” than other colonialism because it was so different. It was not. Colonialism is always a form of violence. And in the case of Angola, that violence although not expressed in the same legalistic terms as Apartheid, still existed (hence why some Portuguese colonists add no problem in transitioning to and fighting for the armies of Rhodesia and South Africa). The stories of arbitrary violence against natives are a common tale, but I think that most importantly for this point, is the structural dimension of this violence. While black people weren’t banned to do certain things, there was societal coercion for them to address white people as masters, to remove their hats when white people passed by, not to talk to white women or children, not to enter their houses and even to avoid attending certain venues or to go to certain places.

You can find an interesting testimony about this structural violence here. The "assimilados", were black people who were civilized "enough" to be entitled to "cohabit" with the white people, they would be seen as "white" and were above the "barbarian" indigenous.

Was it expected? Did the colonists see the "writing on the wall" and perhaps expect what was to come? Or were they blindsided by it?

Yes. From what I’ve gathered, most people already had the desire to return to Portugal, and the war itself lasted for 13/14 years. Angola wasn’t the most violent of theatres of the independence wars of the Portuguese empire, but most people I’ve talked to (and you have to take into account that these are post-factum impressions) saw this coming from miles away, following a general discredit and isolation of the fascist government and the incompetence and ill preparation and equipment of the armed forces, fighting an uphill battle against a overwhelmingly more numerous enemy with the support of the worlds super powers as well as other regional players. That being said, it didn’t make it less painful or traumatic.

Was it "necessary"? In other words, if the Portuguese people hadn't left, what would have happened to them? Would they have continued to live peacefully (albeit in perhaps reduced circumstances) or was the political situation such that they would have been forced to leave in any event?

“Necessary”: no (the MPLA government gave people the opportunity to attain Angolan citizenship under certain conditions)[Source](Marques, Alexandra (2013) Segredos da Descolonização de Angola. Toda a verdade sobre o maior tabu da presença portuguesa em África. Dom Quixote). Advised: probably. Some people did stay, particularly those with some sort of relations to the new emergent power of the MPLA. If you were a harsh “master”, living isolated in the interior of the country, famous for physical punishing your servants and other mischiefs it was advised that you ran. As well as for the people involved in the military and the colonial administration, especially if they had ties to the fascists. Those who stayed, did so under the realisation that they system had changed dramatically and that they wouldn’t be living under the same conditions as before. According to this [Source](Bender, Gerald J. (1978-01-01). Angola Under the Portuguese: The Myth and the Reality. University of California Press. p. 228. ISBN 9780520032217.) around 40k decided to stay behind. But it was also incredibly interesting the “return” of those people to the metropolis.

That million people coming from the colonies all at once, in massive government organized freight convoys, deeply disturbed Portuguese society, already in turmoil due to the revolution and the post-fascism transition to democracy. An extra 10% of population brought to the metropolis facing a wrecked economy… this people didn’t have an easy time. They were looked suspiciously by the rest of the population, both from the left and the right, because they might have had ties to the fascist regime and they were now “leeching” government relocation programs. They also had different cultural habits and some even “darker” skin tones the metropolitanians, and, having being accustomed to be at the top of the social pyramid, they came to a country were such pyramid was being shacked violently and their position was that of just another face among the crowd.

You can find more info about them in this article in Portuguese

Another source about "retornados" - very interesting comments section where people share their stories

I have personal anedoctes about this (as most Portuguese do), but we all know the rules! If you don't get your answer here try r/portugal (for personal anedoctes, not history - that sub is heavy on an "alternative" view of portuguese history).

I'll just say it might help you finding sources if you use the portuguese word that came to refer specifically to those people: "Retornados", or " those who returned".