I spoke with a friend who is a linguistics professor. She's says that forms like inmoral are just plan old dissimilation, and not some kind of learned reverse engineering. (As both Myridon and Faldo say, the Latin forms, showing assimilation of n to m, preceed the Spanish ones.) She's not sure when it took place, but it precedes the composition of El Cid. (She did research on early Spanish phonology.)

Ceci n'est pas un seing.