You can happily do this with a leg press machine where the plate is solid with the mounts. It’s one piece and if it was going to tip over, it would do it as soon as you took the weights off the storage pegs. It’s one unit and does rely on symmetry. I do it all the time with myself and my clients without issue

The Smith machine can be similarly loaded, but many have a mechanism that assumes the bar stays level. It will work, but you could prematurely wear the track, or jam it and break the machine or hurt yourself.

If you’re working with a free bar, it’s advised that you don’t.

Yes, that is pretty much what happens. The creatine can donate its phosphate to any nucleotide, so any ADP that is in need will accept one and can then donate it in whichever energy pathway needs it.