Well if we manage to get there (and it's a long shot now) the main obstacle to anyone who would rather go back to stock trading and widespread destruction of species is that people will see which one works more and if there's a clique of these weirdos first they won't associate with them and second if they infested to a point that they actually mean a threat to the new order, they would just rise up, as it can be seen in South America or Cuba or wherever there is a threat to the system. Anarchism isn't against that.
The other dilemma is a good one and i'm yet to think about it thoroughly, though since it's highly theoretical it's tough to come up with a one size fits all solution. Obviously the main objective would be to avoid conflict. If it's unavoidable (one thing to think about is what outside forces were there in a global anarchist society), i would think that the kind of general solidarity that we see in Bolivia or Venezuela or Cuba would switch on and there wouldn't be a problem about it but i'll think about it (note: this is my subjective opinion about the case you introduced).
Anarchists generally think that state communist parties (council communists are a different question) are not capable to lead towards a stateless, classless society, since they want to use state power, whose primary functions include reproducing itself. I'm yet to read any convincing account about how, if we got there, Leninists would start to break down the vehicle they used to defeat capitalism and rallied society around.
If you're asking my opinion, i have much of the same scepticism towards communist parties, but not on an equal level, for example i see much more potential in Latin American left/communist movements than in China. What i differ from most anarchists tho is that i'd be very happy to be proven wrong and generally won't advocate for the overthrow of the CCP in the current context.