Economic history has no idea as to how far back the divide goes, but it already existed at the time of unification with roots that go much deeper.

The causes are in small part geographic, and in large part institutional. I wrote about that in this long and somewhat rambling answer here, which you might be interested in. I'll to summarize it below while addressing your specific points.

We need to start with the pretext that almost all of contemporary economic development is linked to industrialization. Even the wealthiest pre-industrial communities were less materially wealthy by a factor of two or three compared to communities in the earliest phases of industrialization. So the short answer to your question is that while Italy as a whole was a late and ineffective industrializer, within these confines the North of Italy (which I will call, "the North") was better positioned to take advantage of industrial growth, while the South of Italy (which I will call, "the South") was not. A subsequent slew of misguided and patronizing top-down economic policies ensured the South would never catch up.

The differences between the two halves of the peninsula up to at least the Italian Renaissance were marginal, with the South probably more prosperous than the North more or less until the thirteenth century: with strong ties to the Eastern Mediterranean's florid trade routes to the point of most cities being majority Greek-speaking, the Southern economy was definitely the peninsula's most florid in ancient times, but after the fall of the Roman Empire the South would enter a progressive decline as the land was penalized by interruptions in Mediterranean trade (notably of trans-Mediterranean grain shipments) caused by an increasingly unstable political dynamic in and around the Eastern Mediterranean. This led Italian peninsula's center of population to gradually shift northward as the wide floodplains of the Po river valley allowed the northern cities to sustain themselves autonomously, and rather than cling to ties with the East (be those ties peaceful or contentious - for the South certainly suffered both) the Northern Cities were more selective with their interactions and developed closer ties to the emerging monarchies north of the Alps (relationships which were also equal parts peaceful and contentious, but ultimately more beneficial than those relationships which existed in the South).

According to the estimates compiled by Angus Maddison, the North of Italy was Europe's most materially wealthy economy right up to the Italian Wars of the 16th century. Once the Italian Wars erupted, they not only halted economic growth on the peninsula but coincided with the emergence of Atlantic trade (and colonial trade) which accelerated growth in economies of Western and Northern Europe, while instead leaving the Mediterranean untouched (in aggregate terms). But even as some Italian communities, notably Milan and Genoa, were able to siphon some prosperity off the periphery of Atlantic growth, I would still insist that we are ultimately talking about a marginal difference in material wealth in the North compared to the South right up until industrialization. We can't know for sure, as recordkeeping in the South was pretty bad (unusable to make any reasonable estimates) and this alone objectively does probably indicate less florid pre-industrial social and economic life compared to the better-documented North, but that in and of itself should not be a characteristic disqualifying industrial growth: Much of Germany, parts of France, and parts of Scandinavia achieved industrial-era prosperity without having displayed signs of particularly florid economic development in the centuries prior.

The issue is that if conditions for industrial prosperity do not exist organically (which in Industrial Europe, only really existed in Great Britain and the Low Countries) voluntary steps need to be taken in order to stimulate industrial growth. But as the leading European economies industrialized, political decision-makers in the Italian South were instead declaredly uninterested in industrial development. Sure, among the general trend of indifference there were undoubtedly some who were interested in taking the entrepreneurial initiative: in the Papal States, landowners in the Province of Bologna formed associations interested in increasing agricultural productivity, while a small industrialist class did emerge in the Kingdom of Two Sicilies, centered on the city of Salerno. But given the hard numbers on economic development (examined in-detail in the answer I linked above) by and large most of the capital-owning class in these places felt no need to invest in industrial development, and the political class felt no need to incentivize them to do so: in the lead-up to unification, the South lagged behind the north in terms of railroads, financial institutions, literacy, and pretty much everything else you need to kick off industrialization. This does not mean that the South was bereft of skilled artists and artisans, wealthy aristocrats, or a small bourgeoisie (Rome and Naples were two of Italy's largest cities) but it does mean that industrial economic growth just did not exist, the capital-owning class was happy to live in a predominantly agricultural society, and the political class also did not see the advantages linked to widescale industrializations.

The political leadership in the northern cities, on the other hand, had long been conditioned to exist in a fragmented and competitive environment which when presented with the prospect of industrialization led them to take different decisions. In sprite of initial indifference to industrialization which would never be wholly overcome (even the North was ineffective industrializer compared to the rest of Europe) there emerged a core of political and industrial leaders who championed modern economic development and the Northern political and institutional establishment revealed itself much more predisposed to taking these ideas onboard. In the politically fragmented north, beset by war since the 12th century and foreign intervention since the 16th, trusting results-oriented government and quickly adopting new ideas was an established and vital strategy for survival. Even as much of the North fell under foreign domination, this also meant that states like Genoa and Milan's ties to European hegemons linked them to the wider events occurring in Europe. It's worth admitting that the North's fragmentation and ultimate foreign occupation definetely had more drawbacks than benefits (the mercantile cities of the north had already developed social and economic ties with the rest of Europe long before foreign domination, and to boot the South was also in a Personal Union with Spain for much of its history with much less to show for it) the politically active class in the North was nonetheless conditioned to look at new economic opportunities which a much different eye than the southern monarchy, which instead had spent the centuries was increasingly isolated from the rest of Europe and did not feel the same urgency to develop competitive and critical institutions able to seriously take onboard the idea of fostering industrialization. So while the North did not industrialize particularly well, the efforts of northern entrepreneurs were more effective and better supported than policy compared to the South (again, this is explained more in-detail in the linked answer).

After achieving unity, the Southern ruling class was funneled into the government of a united Italy. In the ensuing decades, the political line by Southern politicians across the political spectrum quickly acknowledged the increasingly evident difference in economic prosperity between North and South, and pursued a largely unified line asking for public sector intervention in the southern economy (I mentioned the very fact that the "Southern Question" was itself postulated by Southern politicians in a somewhat more concise answer on the north-south divide that I probably also should have linked above, which you can read here). But while the chosen solution of public sector investment did help in some respects by improving the standard of infrastructure, education, healthcare, and some government services, it would be taken to a damaging extreme in the mid-20th century: in the decades prior to the Second World War, autarchic policies were implemented favoring established industries (unsurprising given the corporatist, thus pro-Northern Industrialist, nature of the fascist dictatorship) neglected the South save for large infrastructure projects which while widely propagandized, represented exceptions rather than the norm. After the War, public investments ordered from State-owned enterprises (much of the economy was nationalized in the years straddling the conflict) contributed to some initial growth in the south, which however petered out by the late 1960s as the public investment saturated the economy and saw southern entrepreneurs not only squeezed out of productive and lucrative economic activity, but also subject to an economy of dependence whereby workers and entrepreneurs found it easier to follow public subcontracts rather than develop autonomously productive activity, or even engage local government to create more favorable entrepreneurial conditions (e.g. rather than legal infrastructure and educational infrastructure, southern politicians and entrepreneurs lobbied for direct investment).

I have a few suggestions for reading on the topic in a reply to this post.