Generally I would go with unable to attack since you can't meet both requirements at the same time.
Maybe if you were polymorphing into an animal form that could reasonably assume the stance in question - thus transforming the animal form's attacks into the monk stance attacks.
I expect that different GMs would have different opinions on this.
RAW you can't strike unless you have an animal form whose attacks are the falling stone attack (which you don't).
As a GM I might say Animal Form takes you out of most stances (except ones that thematically should work together, like an animal stance and that animal) or pick whichever attack I think makes the most sense to use in the form and say that is the one you use.
Hmm, I would, as a matter of convenience if I was planning to multiclass, pick one of the more permissable stances (the ones that say you "can make" not "must make") so that I could punch with my great ape fists and still benefit (at the moment, this seems to be Dragon, Tiger and Wolf *but wolf gets no benefit* and not Crane or Mountain).
Further reviewing, Dragon Stance is the only one that gets all benefits as advances to Tiger seem to require Tiger Claw strikes.
Ironblood and Tangled Forest (DR or area denial, nice with reach, I must say, are later good stances that are permissive).
So thoughts: Go Dragon/Ironblood or Tangled Forest.
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