Welcome, on board, Stumblerette! I guess I'm not thinking of the new members of Team Verb as doing 'guest posts' but as being regular part of the team.
But to business -- your very fine post on our impressive state fish. I would have been very happy to buy the folk etymology of 'elongated mask' for this fish -- the (apparently public domain) image at the left doesn't show it that well, but I can believe somebody would see the face as stretched forward (compared to some fish, certainly). That is, it's not elongated vertically, but horizontally. I guess that's what makes good folk etymology. ('Sparrow grass' for asparagus never made much sense to me, by comparison.)
We need input from some of the Algonquian specialists on the word, surely, but as a fan of all things Wisconsin, I've always read Leonard Bloomfield's work on Algonquian. (He was born up near Sheboygan, was a grad student in German linguistics at Madison for a while, and of course did incredibly important research on Menominee.) In his famous essay "On the sound-system of Central Algonquian", he shows a regular correspondence between Proto-Central-Algonquian *l and n in most of the Central languages. He says other languages in the family typically have maintained l. So, I'm figuring this is probably a case of distinct but related words being in the mix.
Interestingly, the Oxford English Dictionary Online regards this as likely dissimilation. I suspect they are wrong on this one, and they should check with a real specialist in Algonquian historical linguistics.