The number of times performed and last date performed are determined by the shows that a song appears in, so you could "reset" those by deleting all your past shows.
But if you want to keep a record of past shows, it would be better to copy your database first, then delete the shows in the new copy. Then you could still go back to the old copy if you want to see the shows you performed with the previous lineup.
Note that the last date performed will be updated as soon as you perform a song with the new band, so the only thing you'd really be gaining is resetting the number of times performed.
For your second question, there's no way to set up a "rehearsal" separate from a "show," but you could name the show something like "rehearsal" with a date, or "release show rehearsal" if you want to be more specific. The downside is that this will inflate your number of times performed for those songs.