I'm not really sure about undercover and dial a murder, but in both episodes someone tries to kill columbo: Mike Weinberg and professor Mason respectively, could it be that?
1. What do Lovely but Lethal and a Stitch in Crime have in common?
They also partially share the same set! Take a close look. The lab in "Lovely but Lethal", with the unique observation windows overhead, is the same room as the Operating Room in "A Stitch in Crime", where the observation deck is used.