Limousine would mean a car service, rather than a more heavily populated area's taxi service. Eespecially to the airport as was the case, and typically a Lincoln Continental or similar at that time. So not a "limo" that prom kids are packed into or for celebrities arriving at the red carpet.
It only says to me he didn't have a ride otherwise and got a car service. But it also hints at a plan (especially if ordered for pickup somewhere other than his house; or to the house, but at a time when he knew no one was home) and obviously intent to go somewhere via plane or for a meet up.
Assuming it was considered a legitimate sighting, and without more info, I don't see any reason that he's not still alive (or was initially and died unrelated to the 'disappearance'). If he was late teens when he left and had had only cash-paying jobs (or none), he wouldn't necessarily have a SS# to track.
There's not enough detail to question a decades-late MP report. Perhaps family thought (or was told) it was filed when it hadn't been, maybe one member thought another member had filed it and vice versa, maybe LE wouldn't take a report for myriad reasons, or maybe family knew or figured he wanted to leave, was capable and possibly not close to his family, and that was that. As for why the 2014 report was filed, remaining family could have realized the earlier snafu. Or a relative not even born when he went missing who was curious after hearing stories about him or otherwise stumbling upon his existence.