I tried to find something in the match racing / team racing call books (since they specifically cover starboard roundings), but I can't find anything that's directly applicable. They have their own versions of RRS 18 which may not be applicable in fleet racing.

There's a principle in match and team racing that whomever gets to the zone first "owns the zone" and I suspect that applies here, although determining exactly who got there first could be a challenge.

The summary of US Sailing Appeal 97 says that when there is no obvious "inside" or "outside" boat, then the rules of part A and B apply (essentially rules 10 - 17), although the diagram of the incident in Appeal 97 looks nothing like this situation.