While I think that all of the options are viable, quality choices (thanks to both the community and Plus ^__^) I'm going to have to go with Auto Magic Coat.
Basically, I was nonplussed with Poison Heal. It does nothing to deter non-damaging effects other than status: its anti-status effects are something we already know about, and instead of "stopping" secondary moves, it either absorbs them or makes them pointless. I just don't see this making CAP9 *change* anything--other than making a new, different-typed, higher-BST Breloom.
Guts, on the other hand, works...but again, only for status. Nothing would change for the many, many Secondaries out there--abilities which deal no damage and instead hobble (or even cripple) the opponent. Trick, Curse (from a Ghost anyway), Taunt, Leech Seed, and to a lesser extent Torment, would all still sail right on through. Instead of actually stopping the Secondary, Guts would just shift the focus of which secondaries are used, putting even more emphasis on the non-status-inducing Secondary moves.
AMC just fundamentally seems to synergize with the CAP9 concept better than Guts, and choosing it over Guts will ensure that Flame Orb sweeper sets do not completely dominate CAP9's usage.
If it's decided that a second ability is acceptable, I'll definitely be considering Guts long and hard. My main reasoning for that is that AMC's fundamental philosophy is "the best offense is a good defense." It defends and retaliates in one stroke. Guts inverts that--the best defense is a good offense. By powering up your offenses, you do smash the *hell* out of status-users and (hopefully) stop the Secondary from popping back up again. That accomplishes the mission, just in a different (and, in my opinion, somewhat less effective) way than AMC.
Additionally, by having the two of them together (if AMC is a 'silent' ability, meaning it doesn't announce itself) people will have to really, really think before they try anything: AMC would make them waste a turn if they try to drop a non-status-inducing Secondary, and Guts would make any status-inducing Secondary a figurative suicide move. However, that choice is up to Plus and the community.
Final note for the question about why supporting posts don't line up with votes. I can honestly say I'm almost afraid to post *anywhere* that isn't flavor-centric, because I'm always worried that I'll be breaking the rules, sounding like a total moron (since I have so little experience), or I feel like all my points have already been stated and therefore anything I say will be slapped with the "bandwagon" label and ignored or deleted.
TL;DR: Auto Magic Coat helps CAP9 do what it's supposed to better than the others. Guts works too, but should only be available as a secondary ability IMO. AMC and Guts both being options would be great, but that's not a decision for me to make. Finally, a lot of people (including me) are scared to post because we don't want to break the rules, bandwagon, or look stupid.