Didn't Stannis already legitimize him so he'd follow him into war? Even if he hadn't, it wouldn't take much for Onion Knight guy to say that he was. I'm not 100% sure how legitimizing someone works but they've set up a couple of ways he could be a Stark in full name without anyone questioning it.
I recall Jon refusing Stannis' offer at least twice, because it would have meant reneging on his vow, and Jon is all about that vow.
As for resurrections, it's not just the one unimportant dude:
Kal Drogo was resurrected, but into a catatonic state. It was shown in the first season that it was possible, but the process could be poisoned if either the sorcerer/ess wanted it so, or perhaps depending on the method/type of magic/type, or of person being brought back.
Also, with the Mountain: another person brought back from the dead in an essentially crude but functional manner. The knowledge to do it seems to have long existed in that world and is accessible by those who seek it out, but caution: results may vary.
An interesting issue that bringing Jon back brings--though we don't yet know what kind of state he will be in--which god is the true god? Is the LoL for real? It does seem that in the one case we now know for sure, those resurrections are fully legit and seem to require very minimal knowledge (that one dude was not a priest and never understood the words, but always able to bring his pal back in mostly the same state--and now Firecrotch seemed to be putting her best guess forward and it happened pretty quickly).
If Jon comes back as Jon 2.0: just like Jon Snow but with Kung-Fu grip, then I guess we know that the LoL is the real shit compared to the other gods; accepting maybe that many-faced fellow.