Are you saving to disk as "Song and Sound" as opposed to "Song only"? If you save as song&sound any part changes you make will be saved.
You can only use 1 drum kit, but you can devide it among different tracks. It will all come from midi channel 10, but it can play on several tracks.

You can record layered performances. Just pick which layered perf you want to use and start recording. If you want to isolate any track, you need to put it in single. For instance, if you used a layered perf, but wanted to add drums, put it in single, record the drums, then put it back in layer.