All very interesting. If it was a bad head gasket or leaking valve seals, I couldn't tell by its performance. Started easily, ran strong, perfectly consistent. Until its last ride.

Maybe it was in trouble for a while. My 20 minute drives (three the morning before it died) may have been fatal if they were longer, but the hours between those drives let the engine cool off or drop from elevated pressure.

Tomorrow, the outcome is decided. Two people replied to my offer to sell the car as-is. I'll also see if the shop can get the engine swap price down to ~$3500. Whichever happens first, wins. And if neither happen by Wed, it goes to the scrap yard, sadly.

Thanks for your interesting insights.