Oh hun. I can see you're worried!

I'd push DH on the mattresses on the floor option. He's an adult and able to talk through any issues he might have with this. The littlies are not. He can control his frustration, or go into the spare room if he finds it really unworkable some nights. If the new baby is happy to sleep beside the bed in a bassinet, then you can reinstate your current routine, but if new bub wants to sleep with you, with both of them close on the floor your problem is solved.

I'd stop stressing about trying to control the small peoples' attitude to sleep and start working on the bigger one! It sounds like you came up with something that will make your life easier and work for them. As long as DH can see this is only short-term and won't be forever (maybe not for very long at all), I think he can probably get his head around it. If he tries. GL!