Well, I think where most people get into trouble on the ground is they don't understand physics, and Newton's first law to be specific.
Go look it up... I'll wait.
OK, once movement stops, you are now in a pinning game. It is VERY hard to escape pins, do sweeps, etc, if there is no movement. That first law works against you. Think about the stand up game for a moment. How hard is it to throw someone who locks their arms, squats, and won't move? Its so hard there are rules in Judo against taking that position. BTW, its why most sweeps fail.
The sitting back to back drill is VERY different from fighting off of our knees because it adds the dynamic of motion. It makes that first law our friend. Sadly most people do not use it, they just turn and get to their knees and we are back to fighting from their knees.
Of course, I am rumored to know nothing about this next part, and it has been claimed that to even speak about it causes severe trauma to my id, ego and super ego....
...but before I write more, two words... rubber guard.
I don't understand why you would think sweeps are any different than anything else. Throws rarely work in isolation. Takedowns rarely work in isolation. Submissions rarely work in isolation. Escapes rarely work in isolation. KO's rarely work in isolation. Why expect sweeps to work in isolation or expect them to work easily against trained opponents.
Proper kuzushi in sweeps comes about the same way as proper kuzushi in throws. Yes it may be harder because a more sturdy base can be had on the ground but the same principles apply. Athletes can often force throws when it takes great skill to do the throw with good kuzushi. The same is true of sweeps except it is harder to force them.