I don't want to repeat myself. You will destroy the security of the coin by enabling the hashrate attacker to steal other people's coins. This was not possible before (rather before an attacker could only attempt to double-spend his own coins), thus such an attack was not well funded. You are proposing to fund 51% attacks. I hope you understand that hashrate is rentable and the implications of that. Especially when you are proposing to exchange between altcoins which have very low hashrate security (note Alice's multi-sig occurs on the altcoin not on Bitcoin).

Sorry the cut & choose proposal must be scrapped.

Stick with the CLTV on both block chains where the destination addresses are hard-coded and thus can't be stolen by a hashrate attacker. That is technically sound.