however, when I cast it, I get the "Must be in Cat form" message, but it puts me into Cat Form, but no Prowl. But if i click it again, it puts me into Prowl.
you can't do it with 1 click but this macro shifts you in cat form and allows you to go stealth if you're out of combat with next click. In other words, you have to spam it as fast as you can to be able to stealth before getting in combat. It will put you in cat form no matter what though.
These ugly macros make me sad. What you want is not actually possible because shifting to cat form and casting prowl are both on the global cooldown. It can be done with one button, but not in one button press. Hit it once to shift to cat and then again when the global cooldown has expired.