After the torture your brother (do a DNA test to be sure) inflicted , it's very likely it did seize up.
And when it cooled down, the piston gap came back so it would sorta turn over.
The potential killer part of this is that you have Nikasil barrels. Depending on how the pistons scraped along before they froze, you may have flaked off the coating and ruined the cylinders.
The only way to tell - and my recommendation - is drop the heads and pull the cylinders. Clean them up and look through the bore into a light. Look at ALL the cross hatching for small pockets..like inverted dimples. They may be in a circle around the bore where the skirt skidded to a halt. If the surface isn't perfectly consistent and smooth, the rings won't seat and it will ALWAYS burn oil..itwill run fine for a long while..but smoke like a destroyer smoke screen.
Also run your fingernail up and down the bore at a few places. If you feel a ridge line, your cooked.
Look at the pistons and see if you have any vertical grooves in the skirts. Run a new ring into the appropriate groove to see if the piston has collapsed. You might also tell if the ring doesn't rotate around the piston like it should.
Also look at the heads when you take the valve covers off. If the front towards the exhaust is a golden or dark brown, you've cooked the head and probably needs to be planed. Also look at the valves to make sure they're not cracked.
Good luck and tell us what you find..about your brother too.
If the heads are OK, you might get away with new pistons/cylinders, gaskets, bearings, etc. for around $5-700 depending on how good a negotiator your are. Add another $400 if the heads need a rebuild.
BMW Club of Northern California, Ambassador, BMW MOA, Vice President Vintage BMW Club