When you make a mistake immediately press <ctrl-Z> to backtrack step-by-step. Same as EDIT | UNDO.
Of course. Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. Then you can restore a project to the state it was in when saved. Just double-click the VPJ file (which isn't playable).
Audio clip length is normally sized by dragging the clip edges, A [ or ] bracket will appear. As for speed, there's no fill-to-fit option. Look at the video length, then in the audio speed dialogue Duration field, closely match that length.
If audio and video are linked, then editing one will affect the other, to maintain sync. Right-click either and UNlink, if that's the case.
To prevent edits from affecting other tracks, right-click to the left of a track and LOCK TRACK.
There's no way to globally remove gaps. But if content is on track 1 they are best seen in Storyboard mode. Right-click in a gap and CLOSE GAP.