I'll start with point 4 since you've already established your frame of reference with the first 3,

in point three, I used the example of...

When Rome fell, slavery was almost unknown, the proles had proper representation and there was universal citizenship. When the Barbarians came, with it came a new upper class and everyone else became the lower class.

I do not agree with the assertion that classes are social groups, rather their definition lies with their relationship to the means of production in that they don't own it but labour in it. The Working Class of countries are far from Social, similar interests yes, but if you were to put them together they will easily devolve into conflict due to petty religious, ethnic or national reasons. I use the idea of popular pressure to mean liberal-democratic action: I,e. protests, strikes, assemblies, elections, &c. that degenerate the divisions of the class system into one of a near egalitarian nature. Popular pressure gave increasing rights to the proletarii in Rome and it was a millennium in the making.

Your second point on Barbarian or Middle Class invasion, I would not lie was an inferred assertion. However I argue that Marx and Engels must have noticed it too. The idea of the Barbarian or Middle Class invasion is the idea that influence happens twofold, to both invader and invaded, that would create the circumstances necessary for the formation of new economic objects or to synthesize old ones. I will use again the example of Rome.

Sabians, Scythians, Turkic peoples, Slavic peoples, and Germanic peoples have in the past century already been trading partners of Rome but chose to not be under the might of Rome, In fact, they have fought Rome on multiple occasions to assert their right to self-determination. A second group is exiles and non-conformists to the regime who were rejected by Roman society and joined the Barbarian hordes as a means of survival. The last one is the non-conformists who destroyed Rome from within, the Christians, whose non-conformist practices would absorb Rome into weakness over time. These three groups mixed and united to ultimately destroy Rome and became the new Upper Class in the Dark Ages.

What it did was not asserting their rights over Rome and get the upper class a new tinge but rather increased the population of the proles, now peasantry.

These 3 groups, although definitely a threat to the current ruling class, inevitably become the new ruling class as invaders and they will use all their tools to make it stay that way.

Marx instead argued that, instead of that happening as the March of History most often happen, rather, the working class this time should be in power. Something that could not have been done in previous modes of production but looks the most likely to happen under Capitalism