Here's the thing for me.
English borrows words all the damn time. And their spelling shifts as much as the pronunciation.
While the original French definitely lacks the n, and is pronounced different as well, the n is in there, depending on the specific French accent.
Go visit chef Jean Pierre on YouTube. The whole "onyo" thing they do. The man is clearly not saying on-yo, with no other sound at the end, but there's people that think that's what he's saying. He ends it with that nasal n sound.
And, some French people pronounce restaurateur with that nasal n thing as well. Not all of them for sure, but it does exist.
At some point in the past, it would be accurate to say that restauranteur would be incorrect, because it simply wasn't the accurate spelling for the word. But now? It's an alternate spelling in English, for the word that is pronounced with a regular n sound in English.
Correct and incorrect don't really apply at this point, it's standard and alternate
Now, I'm fully behind Wodinsday coming back :)