Mr DeckerMs Decker was seen hanging out closely with a background character in Managing Murphy's Law, who appears to be called Josh. Could this be Mr Decker? Elliot did briefly interact with Josh in World Without Milo without any acknowledgment. I don't know if we have enough evidence to say with any certainty but thought it was worth mentioning.
Whaaaaaaaaaat?! Man, this was worth mentioning! I hope it's a thing and not a one-off drawing, Josh looks cool. Although there's a good chance it's the latter, especially with that other interaction you pointed out. Your strangely keen eye and awe-inspiring dedication to background characters never ceases to impress me.
Keen eye PhinFerbFan5! Seriously, I hadn't even noticed Mrs. Decker, let alone Mr. Decker!