Actually, the last link is essential reading.
It raised a question or two on some issues I hadn't considered before. The first one relates to back-ups. Lightning network hubs: what's the fail over plan? Does data just get lost?
What happens to MNs when they are holding a transaction and one or more gets taken down? Nothing, I guess. The MNs just select another group to complete a transaction, because users funds are still in their control during mixing.
Hmm. Interesting.
Not sure if you wrote that wrong accidentally, but masternodes are never in control of user funds, the coins never leave the wallet, they're just "rearranged" by the masternodes.

I think if someone pulls out of the mixing process, they might still be charged collateral, unless that's changed? At that point, I think the mixing round failed and is abandoned to try again next time.
Also, I'm wondering if we can't have several masternode winner lists and several mixing sessions at a time? or if not that, do we already, or can we make it so that the mixing sessions are for all denominations at the same time?