the original word DIDN'T have a spoken 'r' in the second syllable when it came into the language, so it is extremely unlikely for the 'r' sound to have been added, since, as you point out here

I haven't looked at the citations, but how do we know this? All sorts of things happen to words after they've been borrowed. Middle English parfit became Present-Day English perfect. Somehow, I doubt it has anything to do with rhotic and non-rhotic dialects or orthography.

Ceci n'est pas un seing.