First off, glad to hear you're OK and that you didn't bash in to the center divider. Same thing happened to me in a 1969 VW bug, middle of nowhere, similar seat dropping sensation and then the rear wheel went speeding past me down the highway. When I tracked it down, all the lug bolts were inside the hub cap
. Turned into a 1/2 hour delay.
Les Schwab was to blame for that one. Add them to the list if they haven't been already.