Edvinsson can get at $10 million+ AAV because he is offer sheet eligible and is really really good. He can leverage that, and unless he is absolutely adamant that he does not want to leave Detroit, his agent should be out there working all of the teams that could offer him massive bucks.
Nikishin's cannot negotiate an NHL deal with anyone except the Canes at the moment. His only leverage is the threat that he holds out or returns to Russia. He also only has 1 NHL season under his belt, a history of concussions, and he is a year and half older than Edvinsson.
I would rather have Edvinsson than Nikishin if that was the choice to be made. Edvinsson is massive out there, has a very long reach and freakish speed for someone that large. Nikishin probably has more offensive upside.
Detroit I'm sure doesn't want to have Edvinsson get a $10 million+ AAV, but unless Edvinsson is 100% beholden to Detroit, I think they are going to be forced to pay him that because of an offer sheet or threat of an offer sheet. This isn't Pavel Mintyukov (4 years $7.2 million AAV), Edvinsson's size and defensive game put him way ahead of Mintyukov.