3) Double-move must be declared, and it is impossible to take two of the same move actions in a round without declaring that double move before you begin moving. This means you can walk then decide to shift, moving less than twice your speed or walk, then run, moving /more/ than twice your speed, but never, ever, walk, then decide to walk again, moving about twice your speed.
Right - I already stepped that through in one of these threads somewhere.
It's similar to 3E, where a choice you make at the start of a round can prevent certain other choices later in the round.
For example, in 3E, the first square you move out of is not considered threatened if all you do in the round is move. Normally, I can move 30 feet, and then I can stop, or move another 30 feet (taking a second move action in place of a standard action), or do something else - say, shoot a bow.
But if I start adjacent to an enemy, and move 30 feet without incurring an AoO, my choices are now stop, or move another 30 feet... but I
cannot shoot a bow, because if I do, then the square I started in is retroactively no longer unthreatened, and I
did provoke an AoO 30 feet ago.
(This was rationalised in 3.5 with the Withdraw action.)
If I have a BAB of +6, I can make two attacks at +6/+1 with the Full Attack full round action. I can make one attack, and then decide based on the result of that attack if I wish to continue attacking (in which case I'm taking a Full Attack action), or move (in which case I'm taking the Attack standard action, and have a move action remaining). Which action I was using to make the initial attack is dependent on what I do afterwards.
However, while I can make three attacks in a Full Attack action if I use the Rapid Shot feat, I can't decide to do that after taking my first shot at +6, even though I can elect to make a Full Attack action at that point... because if I were using Rapid Shot, my first shot would have been taken at +4, and it wasn't.
Having later, normally valid actions forbidden by earlier choices is not new to D&D.
-Hyp.