There's been no confirmation that either Jiren or MUI Goku are stronger than Beerus, so no, it's headcanon.>muh stronger than a GoD
Yes, Jiren is stronger than Belmod, who again isn't confirmed to be stronger than Beerus.>muh arm wrestling
Is not an indicator of Belmod being more powerful. Particularly in the manga, Beerus is clearly one of the strongest if not the strongest GoD. Head. Canon.