1. For me, when I record 'over' it just adds a new lane on-top-of and I can double back to that track and delete the new lane.
2. In Piano Roll View (PRV), you can right click and drag to select a group of notes.
3. to move a bar and replace it with a different bar...
- Move to the start of your bar you want to remove and hit s for split (where the selected item for edit is the clip)
- move to the end of the bar and hit s for split again
- drag the clip to a new place or delete it
- drag highlight the section of clip you want to move to the new spot and hit ctrl+alt+c (copy special) to control what you copy -- this isn't strictly necessary but you should know it's a choice over ctrl+c
- move now time to the beginning of the measure where you want the clip pasted
- paste in the new clip
- you might choose to fade clip ends by dragging in the right and left clip handles if you want
** note that this is totally not the only way to do #3. I'm trying to make positive sure you don't hear the original clip. From why you wrote, it should have been sufficient to paste over. Also consider that you could add a new audio track and paste the clip there then drag that clip into a take lane on the original track. That would give you more control as you could see both the original and the new.
Since you are heavy into Midi tracks, you might seriously consider buying one of Scott's Sonar Power books. They're really helpful. There are all kinds of great midi tricks and computer shortcuts that I can't always remember. Midi tricks is one of the main things I open up Sonar power for these days.
Welcome to the forum.