There's nothing implausible here.  I'm presenting a realistic scenario where the coordinator operates under chain analysis without Alice, Bob, and Charlie knowing.  It's entirely feasible for a chain analysis company to trace Alice's inputs and launch a sybil attack against her. 

Your "solution" relies on Alice interacting with strangers, and it still doesn't address the core issue.  The coordinator could use the excuse that their round was already full before Bob and Charlie requested their coins to be combined, or they could claim they 'randomly' assigned Alice to a separate round from Bob and Charlie.