1. Yes, pretty much anyone will work because you are planning on replacing them very soon
2. Yes, farm phoenix even on the side while focusing on empire
3. A. If you are referring to clone sergent definitely stop. At least for now because his ship is only marginally useful I have ditched it for now.
3. B. Dooku is a very op character in the beginning and will help greatly in arena. For a really long time I would refresh until I found a team without him because the counters and stuns shut all of my teams down. He will help with arena rank at least and will synergize well when you get palpatine.
4. Look at the leadership ability of the characters that you are using (not all have one) and see what that says, but yes staying with one faction is best for teams with a few exceptions for endgame players.
I hope this makes sense and helps.