@Findegil - apologies for not responding sooner. It's been a hell of a year for me!
To your questions
1) Wang Men Mu was indeed designed to allow him to use Rabbit Fu solo. We purposefully wanted to streamline the rules so combo'ing was a sequence of actions that fully resolved and lead into the next one, as opposed to being an entire singular action that had multiple cards being played. Chief among this was that it helped newer players since it was extremely confusing for people why shootouts would end if the opposing posse "vanished" during Shootout actions or Resolution actions except when Rabbit Fu chains were occurring. In terms of other combos, he wasn't designed with that intention in mind, but that's not to say people won't figure other things our or future cards won't potentially interact with his ability.
2) Regarding Deeds on Value 5 being strong shootout oriented, yes that is intentional. Values A-4 tend to be fuel for Legendary Holster, which is a very powerful shootout action so it doesn't really need more gas so to speak. As you increase values (usually 7 or above), they become more resilient to negative effects and help with fueling pull decks. This leaves value 5 and 6 as values that don't tend to make either cut so those abilities tend to be very powerful to compensate a little for the fact that 5 and 6 tend to be difficult to run if your deck is based on making pulls.
3) Regarding Wendy XP's wording - it's been a long time so I can't say I remember clearly anymore. I think there was an wording issue that came up regarding effect sequencing since we were trying to streamline a lot of the rules at the time and Wendy's wording was constantly being adjusted to account for this. It's possible that where the rules ended up versus the wording that ended up on Wendy XP didn't end up aligning, which would be my fault as lead designer at the time (and no one else's). Looking back on it with where the rules ended up, something akin to "Shootout, Boot: Choose an opposing dude. If they have bounty greater than their grit, discard them. Otherwise, send them home booted." would have made more sense.