While I will avoid the first question, due to merely knowing less about the post-Norman Conquest period, I can answer the second to some degree.

The Anglo-Saxon (And Jutes, I will use Anglo-Saxon to refer to all three groups for simplicity's sake) Invasion was substantially different from the Norman Conquest in an important way. Where the Anglo-Saxons settled, roughly in the area which later became England, while the native population was not wiped out physically (Genetics prove this, and there were simply too many of them for men with swords and spear to have committed genocide against the natives on anything like the scale we know today), their culture receded into Cumbria, Wales, Cornwall and Pictland. Old English became the majority language, what burials survive tend to be Anglo-Saxon in style, houses began to be constructed in the Continental pit-house style... These are merely some of the things that changed during the period.

Essentially, a relatively small group of Germanic invaders, taking advantage of the chaos in Britain, had somehow managed to convince the locals by force or otherwise to begin taking on their customs, clothing, deities, and language, necessitating a later reconversion of the area by missionaries who coordinated with what survived of the (Rather unhappy and discordant) native British church.

Now, contrast this with the Norman Conquest. While changes were no doubt enforced on the newly conquered kingdom, and the language did change in some significant ways (Something which it seems Tolkien didn't like much), an awful lot stayed the same. William exploited the system of reeves and shires made by the Anglo-Saxons for extracting his taxes. Domesday Book would have been extremely difficult, if not almost impossible, without the pre-existing Anglo-Saxon systems that had already made such things as the Burghal Hidage, a document that documents around thirty Burhs (Fortification/towns), their size, food rents due, garrison numbers etc.

And most importantly, it didn't shatter the kingdom into many different competing kingdoms like the Anglo-Saxon invasion had. The Kingdom of England was an Anglo-Saxon construction, along with many of its institutions (Many county boundaries remained almost unchanged from Anglo-Saxon times until the 1970s).

So basically, while the Norman Conquest did change a lot of things in England and Britain in general, and it did wrench England's focus from the North Sea round to Continental Europe (Especially France), it didn't have as much of a cultural effect as the Anglo-Saxon invasion had.

Sources:
Chris Wickham; "The Inheritance of Rome"; Chapters 7, 19
Matthew Innes; "Introduction to Early Medieval Western Europe, 300-900. The Sword, the Plough and the Book"; Chapter 8
James Campbell, Eric John, Patrick Wormald; "The Anglo-Saxons"; Chapters 1, 9, 10

Note: I still haven't learned how to source things properly, so if these are bad sourcing methods, please just tell me how I can do them better!

EDIT: A word

You seem to be drawing a connection here that does not necessarily make sense: That if Tolkien thought the Norman invasion was bad for displacing Anglo-Saxon culture, he must logically also think the Anglo-Saxon culture was bad for displacing the pre-existing Romano-British culture. But this assumes that Tolkien's problem with the Normans was the displacement itself, and that if one displacement was bad, a different displacement ought to be equally bad.

I don't think you can safely make this assumption. Tolkien liked Anglo-saxon culture for its own sake. He may have seen a displacement that removed it as bad, not because it was one culture displacing another (to a greater or lesser or more or less violent extent), but specifically because it was Anglo-Saxon culture that was being removed. From this point of view, a different displacement that increased or initiated the culture he preferred would have been beneficial, not bad.

Or by comparison, if you liked rock music, and complained when a rock music station changed to country-western, I wouldn't expect you to be equally upset about how the station had initially switched from jazz to rock, even if the change in musical form had been just as drastic.

Didn't the Norse invasions also have a large impact on the formation of the English language? I was under the impression that Norsemen formed families with the local population, merging the languages.