According to the rules, I'd say:
4) You must not play Missiles before starting the actual combat, but only after the possibility of free retreat.
The rules say that the initial Combat Strength is *only* determined by (highlights by me):
Each Fleet has a Combat Strength of three
Each Star Base has a Combat Strength of five
Any cards that modify Combat Strengths (which are Space Mines, Dreadnoughts and Super Dreadnoughts). [i.e. the passive/permanent reserve cards]
This is similar to how the Star Bombs hit-and-run tactic is described: you cannot play Star Bomb before the retreat step; instead, even if you plan to lose the fight, you have to enter the "real" combat phase first, play the Star Bomb then, and take normal loser's casualties.