No than your save games wont work...
You can make a copy of your exe and rename it to gta_sa-sp.exe than you can downgrade the other one, and still play SP savedgames.
Also note that when you are in windowed mode and minimize it, than it will screw the overall sync (no fuel tank explosions, some vehicles wont take damage)
So ALT + Tab will have the same issue than, I think.
But with to much vehicles and the Z is +1 above the ground the vehicles will float in the air.
No it will not. The gravity will take care of that!
Only what happened is that the cars can blow or have damage.
It happends on a server were I made a vehicle map for it.