I'm pretty sure this is covered someplace in the rules.
First, the limited version of Withdraw is in fact available only when the character is limited to a Standard action only in the round, such as when on a Readied Action. So that part would be okay.
As far as the animal always acting on it's master's turn: I see that as a simplification of Initiative rules. The animal won't attack unless ordered to, or released into battle. One battle begins, however, things like Delay or Hold, or buff spells that change Initiative could reasonable separate the two.
...
I just read over the section in the SRD on mounted combat. It says that you can get only a single melee attack in a round when your mount moves more than 5 feet, because you need to wait for the mount to get you there before you can attack.
Sadly, it doesn't clarify whether that limitation applies to ranged attacks. To many, the fact that it mentions the limit specifically w/regard to melee attacks would suggest that it doesn't apply to ranged attacks.
Still, time is a funny thing. If you and your mount are on the same initiative (the default), and your mount moves in response to enemy action, I'd make you pay the price for firing while in motion. But that's just me. Others may allow it with no penalty, while others may disallow the entire maneuver.