Note to self: Flying creatures with claw or talon attacks can perform diving charges for double damage. Clumsy flight makes it hard for zombies to line that up sometimes, but it can be done and that can help boost their damage.
Well, how else would it attack with them all even if it moves?
Any number of ways.
- It could be a free action (or no action at all) for the hydra to make its attacks if it's already done some movement.
- The attacks could be part of the move action.
- It could be a special full-round action to both move and make its attacks.
- The hydra could get a bonus standard action each round for each head beyond the first whenever it moves or charges, usable only to make an attack with that head.
That's just off the top of my head.
You really are making it more difficult than it has to be. It's a natural ability of Hydras to be able to attack with all of their heads at no penalty whenever they take an attack action. Normally, I would say this isn't the case on AoO's, but they explicitly have that ability as well (at least if they have the Combat Reflexes feat).
That's not supported by what the text actually says. It says that they can make multiple attacks even if they move or charge. That's the part that's special (they're all primary natural attacks, so the default case is already that it can attack with them all at no penalty in a full attack), and that's the only two cases it addresses. Can a hydra that neither moves nor charges make multiple attacks as a standard action (maybe it's staggered and immobilized, if you want a situation where it would come up)? You're arguing that it can, but the actual entry doesn't address that at all.
If you have anything from WotC addressing hydra attacks in 3.5 (errata, Rules Compendium, FAQ, etc.), I'm all ears. Until then, I'm stepping away from this line of discussion.