For three nights our LO would sleep from 9:30 to 6 on his stomach. After the third day I mentioned it to our pedi and he said it's actually a really bad idea to leave him at night like that. Reason a) it can build up so much pressure in their chest which causes them to stop breathing, b) higher risk of SIDS.
Our LO would flail his arms around for hours and wouldn't sleep, hated being swaddled and so then we raised his mattress 30 degrees and put him on his back, after a few nights he than stopped moving his arms so much.
I know the joy of sleeping through the night when they are on their stomachs but it's just a better choice if you give in and let your LO get use to sleeping on their side or back!