Playback Stops in the Middle

This is an MS 3.6 score containing two versions of the same short piece. There are a couple of problems, all of which trace back -- I think -- to how it was created. I am looking for guidance as to how to fix it.

Problems:
1. Playback stops after measure 33.
2. The mixer shows four channels. There should be only two.
3. The Instrument dialog shows staff 1 and then what looks like a phantom staff to me.

Creation:
1. The file was originally a single instrument ("treble staff") containing 25 measures - what is now the second tune of the pair.
2. I added a second instrument ("violin", because I could not select treble staff when adding the 2nd instrument.). I changed the order of staves. I copied the what is now the 1st version of the tune from a separate score into the new staff. (I was wanting to compare two versions easily.
3. I appended 25 bars and cut-and-pasted the 1st tune from staff 2, measure 1-25, into staff 1, measures 26 - 50. I did some housekeeping: restoring repeats, adding the frame with the second title, etc., putting in the sego and D.S.,, etc.
4. I then deleted staff 1.

(TBH, I was not logging my steps but the above is what I believe I did.)

Voila! Perfect score! (I know - you are clenching your teeth in agony having read the twisted way I got here.). Perfect, that is, except for the problems noted about.

Wha the heck do I do now?

Thanks in advance!