I applied a little 'control' and 'variable' to this experiment:
I wiped off the bearings and crank, applied some oil, installed the crank and all caps...no turn. So that was 'control'.
I removed all the above, removed the #2 and #4 bearings, reinstalled the crank and caps...it turns.
Whoa! That sounds like the "smoking gun" to me. The #2 and 4 main bearings only receive oil pressure through the crankshaft oil passages. And, of course, all of the rod bearings are oiled the same way. It sounds like the STP created an embolism in a crankshaft oil passage that starved #2 and/or #4, leading to the bearing failure.
The condition of the rod bearings would also be suspect (I don't mean the bearing shells - I know you plan to replace those - I mean whether any of the rod bearings seized too - probably not, as that usually results in catastrophic rod failure).
Assuming your crankshaft and block aren't warped or damaged, the solution is to have the machine shop remove the oil galley plugs in the crankshaft, clean the passages with cylindrical wire brushes, and re-install new plugs.
A better fix is to replace the swaged-in plugs with tapped holes + allen head plugs.
The best fix is to cross-drill the #2 and #4 block and main bearings for direct oiling.
Sorry for the late reply - I've been travelling. Sounds like you already have your crankshaft back. You can try spraying carb cleaner (or some other aerosol solvent that comes with a wand) through the crankshaft oil galleries to ensure that they aren't still blocked.