What they really never foresaw was the rise of a media---which really didn't exist in its modern form in 1787---that would be able to cow and intimidate ALL those checks and balances branches into submission. The powers still exist. We just have no one who will use any of them.
You are right, though, that the horrible weakness of proportional representation is that it allows fruitcake parties like the Greens or Nazis to obtain real power through . . . intimidation and threatening government shutdowns if they don't get their way.
Youre wrong about that.
The USSC invented that power themselves.
The Sweden Democrats party is not comprised of Nazis. This is a canard of the left, especially in Europe, to discredit any party that is conservative (relatively) and supports the preservation of language and culture and a reduction in immigration.