Reply 2 years after so i think you found your own answers to this.
My own interpretations:
1. Nothing in the rules mentions it. I understand it the way any one can play action cards between the declaration of the attack and the firefight cards step without order or any limitation. I assume then both players agree for the beginning of the attacking player firefight card decision
2. There is no limit about this in the rules. Assuming this is then a big big push ?
3. No. You shuffle your action deck. not your action deck + discard pile
4. As i understand it. The Uranus card is mentioned then as discarded -> go the discard pile
5. Two rations to 2 forces means to me 2 rations for each of two forces(=4).
6.Immediately is not 'any time'. So the card can only be played during your own action turn.