I'll probably get to play my first game tomorrow so I wanted to clarify some things.
Does the momentum token ever change hands mid battle? I don't see any reference in the rules to that happening. If it doesn't, then I suppose that the player that holds it gets to move first every turn, unless his opponent deliberately holds back on giving commands.
The generic "Rally every unit" cards feel like they should rally every unit on the board. I'm guessing that's not so, however?
Your assesment of the Momentum situation is spot on. There is no mechanic in the base game for the Momentum token to change hands. The expansions add Gambit cards and rules for Momentum to change hands.
You are also correct about "Rally every unit". It works within the ZOC of a leader. It is a difficult card to master.
Definitely goto FFG's support page for the FAQ, Headless Hollow for the playaid and go through these threads and look for KenToad's Geeklist rewrite of the rules (still under construction).
Good luck and post more questions if needed!
edit=== if you need links to the above resources, let me know and i'll quit being lazy and add them here.