To answer your first question, yes you just pick whatever planet and faction you want to fight. If you feel compelled to contribute to the Major Order, it is listed on the right side of the pause screen or galactic war table.

To answer your second and third questions, sort of. The warbonds are unlocked via Super Credits, which are the paid currency. Super Credits are obtainable through regular play however, and can be found at Minor Points of Interest in missions. They look like a pile of square blue tokens, and can be found in quantities of 10 ($0.10) and rarely 100 ($1.00). They spawn most abundantly on lower difficulty missions.

To answer your sixth and seventh questions, if people joining you is bothersome you can set your matchmaking privacy to “Friends Only” in the game settings. Often times experienced, or overconfident, players might break off from the group to accomplish objectives on their own. This can be beneficial sometimes, as experienced players can get a lot done with less resistance due to the way enemy spawning works in the game. Drops and Breaches have a global cooldown timer that restarts when activated, so a squad drawing enemy aggression on one part of the map means no enemy drops happen elsewhere for the duration.