Yes, a better synthetic ATF like Amsoil or RedLine. You'd need to put a gauge on it and do tests to know for sure. But my transmission definitely shifted MUCH better on the Amsoil (the lousy 2-3 shift smoothed out) and it's all I'll use now.
Additives are a risk: you don't know how compatible they will be with the fluid you're running and so on in the long run. Best get a quality synthetic if you're concerned.
Yes, but it will last about forever. It was worth it to me just to smooth out that stupid 2-3 shift! I wish I'd had my tranmission temp gauge BEFORE I put it in. Oh well...
I can't remember but it's pretty expensive -- like $6 a quart or something. I probably wasted about $10 worth, lol, doing my flush. If you just "drain" your tranny, you'll only replace about 40 to 50 percent of the fluid -- but even that will help.