Is there a specific name for a portmanteau of two alternative spellings?

I'd call it "franglais" (the contraction of "français" and "anglais"), as juxtapositions of these two languages (French and English) typically carry that term.

Another suggestion would be "french anglicism", as the French call an English word, spelling, or pronunciation that is lent into another language "un anglicisme" (French wiképedia link). An example of this in French would be the common usage of the term "e-mail" before (and after) the introduction of its French equivalent, "courriel". This seems to be an example of the reverse (French to English instead of English to French) occurring, hence the suggested name. If you're looking to make a portmanteau of "french anglicism", I'd suggest "franglicism".

If not that, I suggest "word cocktail".

I think you'd call it "spelling mistake".

The error likely comes from the fact that "Esquimaux" is always plural as in "of the Esquimaux", where the singular would then be "Esquimeau" (which is not used). Eskimeau is just a spelling mistake - there's no need to assume that the artist was deliberately trying to fuse the english "k" spelling with the french singular ending.

+1 for Franglais though. We use that all the time round here, estie.

I think you should probably just call it a portmanto or a portmanteaux.