I agree and would suggest simply removing all the forms that are not listed in dictionaries, and indeed, are so uncommon that most teachers and editors would consider them misspellings.
I agree with Cambridge, M-W, and AHD that Mohammedan is the standard English spelling of the historical word; no idea why Oxford lists the relatively rare variant Muhammadan first. Actually, it looks like a bit of backtracking. As far as I know, the spelling Muhammad only became used at all in English in the last couple of decades, as Muslim sensitivities have made the spellings that are closer to Arabic more popular. But by that time, Mohammedan had already long fallen out of use except as a deliberate pejorative, for obvious reasons. So Muhammad + -an seems to me like a weird hybrid, with the old pejorative suffix but the new Arabic spelling. Since anyone who would use the term at all nowadays would by definition be using it in a derogatory way, the PC spelling seems logically highly unlikely.
With other terms related to Islam, the spelling is still somewhat in flux. In AE at least, Muslim is now preferred fairly strongly over Moslem in all contexts, to the extent that the old spelling looks strongly dated and is usually frowned on by editors and style guides, though it's still occasionally used by traditionalists. (And perhaps more BE speakers. I think German speakers have said in the past that the spelling with O is still more common in German too, but if that's not true, you all can fix the German side.)
However, my sense is that Mohammed, as a proper name, is still used fairly widely in the general language, with Muhammad remaining considerably less common except among specialists or Muslims themselves. The same is true of Qur'an, which is less common than Koran.
But all that is really beyond the scope of LEO, so I don't really see the need for an explanation in italics, either. It seems to me that the old ones could simply be marked archaic, and perhaps also pejorative. (Really, they're either archaic or pejorative, but I'm not sure you can get an 'or' in there.)
So my recommendations would be
Mohammedan (also:) Muhammadan [Rel.] [arch.] [pej.] - der Mohammedaner | die Mohammedanerin [arch.] [pej.]
Mohammedan (adj.) [Rel.] [arch.] [pej.] - mohammedanisch [arch.] [pej.]
Mohammed (also:) Muhammad [Rel.] - Mohammed
Mahomet [Rel.] [arch.] - Mohammed
Moslem [Rel.] [dated] - der Moslem | die Moslemin
Moslem (adj.) [Rel.] [dated] - moslemisch
Muslim [Rel.] - der Muslim | die Muslimin, die Muslima
Muslim (adj.) [Rel.] - muslimisch