can musescore edit imported midi tracks?

Once it's been imported, you can can do anything with it, including transposing. Of course, MuseScore is a notation program, not a MIDI editor, so it's notation-oriented things more so than performance-oriented things that you can edit.

Absolutely; this is what I use MS for the most. Often I'll delete all but one or two tracks, then edit them to conform to standard notation. Key changes, octaves, whatever, no problem.

I do find that a lot of hidden garbage comes in with the MIDI, so I always select all of the notes and then set the Note Properties to default value (0 and Auto in the right column) or else the playback can be funky. I wish there were a way to really purge the score of all hidden properties... maybe there is.