I don't understand. A double spend means that I use the same utxo for a coin join and for a normal spend right?

Once the spend was confirmed into a block, the coinjoin should have just failed and asked everyone to restart before it got broadcast.

Either way, full rbf or not, nothing is confirmed until you have 5 confirmations. You can't guarantee nodes won't somehow let the old tx get to a miner who might mine it.

So all coin joins should take a minimum amount of time. Don't fuck around with something so important involving so many people and so many attack vectors.