Popped by for a minute, and some of this describes my DD, who just turned 3 and is still struggling with transitions. (Just tonight we were the last to leave an event.)
She's improving a bit, though my tactic tonight when she refused to get into her car seat was to shut the car doors, turn off the interior lights, close my eyes, and tell her, "Well, I guess we'll need to sleep here in the parking lot." ;~)
When I told her we couldn't drive off if she weren't in her car seat because the police might arrest me, she got concerned, "But then I wouldn't have a mommy!!" And let me buckle her in and give her a hug ;~)
That's more "logic" than would have worked at 21 months, but sometimes distractions did -- and still do -- work. "Hey, look, did you see the xxx over there?!"
Let me know if you discover any other good tactics!