To me, though, the way both that neither /ar/ nor /m/ is echoed anywhere else in the line and that /ar/ is jumps out of the progression of where the other (stressed) vowels are pronounced in the mouth make it jar.

I understand what you're saying; it neither feels nor sounds that way to me. All those vowel sounds are on the same downward plane for me: it's not a jump. This may be a difference in accents or just in tastes. People like different things.