Looks like no help from the folks here on Houzz for your sheet issue. At this point you might have the answer for the rest of us. If so, please post! - Kelly

I am going to make it so the side rail drops down to a 90-degree therefore you can pull the mattress away and have access to three sides

I stood on a stool at the head/foot of the bed to tuck the sheets in on the ends, but with built-ins that may not be possible.

I read somewhere to put theee fitted sheets on there (safety pin the bottom one to the mattress) and then you can just peel them off one at a time when it’s time to wash. It’s still hard to do but you will have to make it less.

i'd be tempted to use a queen duvet, hinged to the back. like an unzipped sleeping bag. you could work out a way to button it to the covered mattress.