So for the Senate to have that power, they needed to have equal representation from each state otherwise populous states would have too much influence on the internal government of other states.
That's part of it, anyway. It's more complicated and I'd recommend you read Madison's notes on the Constitutional Convention if you're really curious.
Because while they were aware of that, there was more at play than racism and slavery, as Virginia voting against its interests suggests.
Slightly pedantic, I could be wrong, but no serious attempts were ever made to conquer south; less than the attempts to conquer Canada, anyway. History is complex!
The Virginians, that aristocratic lot, would have and did prefer features like the Senate, which would have provided a counterweight to the more radical House. It's that way even today.
I can't speak further to the historical context, but everything I've read about federation/modern/consensus democracies suggest that bicameralism is a benefit (ex http://wikisum.com/w/Lijphart:_Patterns_of_democracy).
Again, that's not saying that's what they were considering then, but also read some of the Federalist Papers (Madison in particular) and how much they were weighing factions/balance of power into some of these solutions.
Uh, I'm not convinced it's working here, chief.
On the Convention count, as you can see, I find some logic in both explanations.
Florida and Texas?
It's pretty evenly split. It's difficult to suggest it's all or mostly about slavery.
It may be compelling but claims like that need evidence, and when you pull in the evidence it's not so convincing.