The Beast was given our foam mattress.
We then went to Ikea to look at mattresses there.
There was one that was very soft. Speaking with the person servicing that area, he said that spring mattresses are softer than foam. A semi-firm foam is a firm spring. He was quite helpful. We picked one of the softest there, but with a foam topper. This made it slightly more resilient than the one without. But the one without the topper would have put us down to the foundation. Cost: ~$500 plus $60 shipping.
lilacstar is doing better, but not fabulous, on the new mattress.
Then we made time to return our old mattress.
We went to the store, only to find it staffed by a guy who managed another store. He wouldn't return the mattress for another because it wasn't his store, come back tomorrow.
I called ahead the next day. I explained that we wanted to return our existing queen mattress for a twin and a foundation. He was cool about it. We finally made it there that night. We looked at a bunch of twins, and had the Mouse test them out. The mgr told us we had a lot of money to work with, and steered us to the more expensive brands. Mouse found one he liked. We got that and a bunky-board foundation (very low, for use w/ bunk and loft beds).
We got the mattress, foundation, and a cover, and it came out to the cost of our queen, minus the 10% restocking fee, to the penny. Throughout, the mgr was very nice and helpful. Until I pointed out that our original invoice stated the delivery fee would be waved if we returned the mattress. It was unusual, but we were offered it.
The Mouse's new mattress gets delivered Tuesday.