Mjolnir being broken was never a worry in a fight with Norrin in the past and won't be now. I highly doubt Norrin is capable of breaking Mjolnir and most importantly Thor won't let him.
It's unclear to what extent he was powered up if he received any power up at all. He told Galactus no etc. It's unclear as to what that was referring to but their is a chance it was referring to the offer of more power.
Rhino never beat Norrin. Norrin treated him like a joke when he got serious. Even stopped his charge with one arm when he grabbed him by the horn.
Hercules and Thor are most definitely stronger than Norrin.
Ravenous never beat him. When he first fought Ravenous he was going toe to toe with him and he was holding back like usual as I recall. Asking him to surrender etc. He then gets mad and unleashes his power in a omnidirectional burst. He then flies off with Air Walker. In their second fight after the manipulations of Galactus he went straight to business. They go toe to toe for a bit and then Norrin overpowers and defeats him. That's what I recall at least from the Annihilation Silver Surfer tie in issues.
Yes Silver Surfer was wining but it's not something he would not be capable off before. Norrin attacked him from behind and took the advantage. Seeing as it's a split between the two in terms of power I don't see the problem. On the flip side, Bill kicked Norrin's ass when he took him by surprise.
I have to go back to re-read it but I recall reading the Galactus incident where he changes him and thinking that this could be taken either way. If it was a power up fine, I'll concede.