Well, by the current implementation each side chain would have to do the FCMP within itself, but yes, with FCMP the membership set would include everyone within the side chain. There’s probably a way to expand the membership proofs to include sidechains — I can’t confidently say this is true, implementing this would be significantly beyond my pay grade — but I don’t think it is necessary, it won’t affect Monero’s fungibility. “This transaction came from anyone in the current chain” is large enough to be fungible. In a way Monero’s aggressive delisting has done it a massive favor. Because every time Bitcoin (or any other chain’s) fees have been excessively high, it has been because of the mania of speculative investors and not actual use cases. And the entire actions of Bitcoiners these past few years have been trying to convince some investor of an unfalsifiable claim that villages in South America are using Bitcoin all the time, while keeping plausibly deniable. Bitcoin’s development has been subject to the near exclusive pressures of ideologues or transient hot money. Monero, with its lack of access to the major liquidity sources, has continued to simply address the needs of people actually using it. If eCash proves a market demand for drivechain Monero can adopt it at its leisure, but fees remain low throughout because it only has actual users and not a flood of deranged speculators.