You can easily convert to new Functions, just right-click and choose "Replace with built-in function".
1. Number of undo steps is set on UNDO sprite, maxSteps variable.
3. Yes, if you use System Save somewhere else, it will be saved as undo step, so you need to add some kind of flag, say a boolean variable isUndoSave, set this flag in "SaveStep" function, and check for this flag in "On Save complete", also reset this flag there.