Break an unwanted Tie and enharmonic question

I am new to MuseScore. The version I have is 0.9.6 revision 2148 running on Ubuntu 8.10. (32 bit)

Once a tie is established how do you break it short of deleting the notes involved?

Concerning enharmonics. I assume there will always be issues involving this.

I created a lead sheet in Concert and wanted to transpose it for a Bb instrument. I did a "Save AS" and proceeded to transpose the entire newly created document. It seemed to do that fine, but many of my notes were not enharmonically as I wanted.
1. Is there a way I can work with the app to facilitate it doing what I would like? Case in point, the concert piece was in Eb minor. Transposed it was in F minor. I had a bVI7 chord (B7 or Cb7 in concert pitch). Writing the original I could choose either symbol to represent this. The first time I used B7 in the concert document and when I transposed I got a lot of sharps in my transposition results. I wanted the melody to use as little accidentals as possible so I tried using the Cb7 chord symbol to represent the harmony. I thought this might change the resulting melody enharmonic spelling, but it didn't. So what are the rules?

2. When the inevitable happens, namely you get the wrong enharmonic spelling, how is the best way to fix this? Is there a function for selecting a note(s) and changing their enharmonic spelling? Can you define your own "macro" or key shortcut? Do I have to do what I did, which was slog through the whole piece fixing individual notes by typing their name?

Thanks for any feedback given.

bones12