Hi
I think with what you've said I'd probably not offer the nap until at least 10.30am. If he wakes early (5.20) this makes a long A time but it ought to help move his WU time back to the 6.30 you were getting. To be honest on one nap even 10.30am is too early but perhaps you can move it out after a few days to 10.45 and then more towards 11am. I am assuming that if nap comes 11.00 - 12.30 that you have time to whiz him off to nursery but perhaps your travel time is longer than this - I would just make it as late as you can and still fit in 1.5hr nap, perhaps he can wait until he gets to nursery before he eats so that he can sleep right up to the leaving time?
Once he is settled into one nap and needs it to come later he might change his mind about sleeping at nursery, it's possible that when the A time really stretches he will stay up all morning and enjoy his time with you and then have a sleep at nursery after lunch. When it comes to this you might need to just force it, keep him up all morning and ask nursery to do what they can to get him to nap even for a bit. After a while of this he "should" then start to accept an afternoon nap.
I would try to keep every day the same so that he knows what the routine is. Some LOs are fine with changing routines each day, others not so, mostly I see predictable routines do help. As for MIL doing her own thing, well, deep breath