The purpose of a Sybil attack is to PREVENT Alice and Charlie from participating in the same round. If Alice and Charlie just coinjoined together, that's a FAILED Sybil attack, not a SUCCESSFUL Sybil attack.
If you read my message carefully, you'll notice that my point is this: since the coordinator knows Charlie's coins, he can block him as soon as Alice joins. If Alice quits the round again, to "test" that the coordinator isn't Sybil attacking, the coordinator can continue with what he did previously.
And all those points you're making are complete garbage. It's amusing that you're presenting them as exceptions, when in reality, the rule is that you don't need friends for a coinjoin, and the vast majority of people don't already have private coins before coinjoining. Let alone, export the PSBT and look on the inputs to see if they really register in the broadcasted coinjoin.