"But the question remains: how did they get in? We can't be sure, but there were several hints that Enid was one of the Wolves, and that she may have been working with Ron to sneak them in."
How did they get in? It doesn’t appear to be that difficult because they built the butresses on the outside of the walls. Just climb on up and go over. We’ve seen Enid do it.
That explains why she almost turned around at the gates of Alexandria. Guilt.
I don’t think Enid is a part of the Wolves. She showed up, dirty, hungry and catatonic after her few weeks alone on the outside. And Ron is just an angsty teen, living in a boring but safe housing development that managed to be safe in the ZA until now.