So much depends on quality information about proper engine
operations, yet there appears to be little science behind the assertions.

You are quite correct - there is very little science here. There is
certainly a lack of solid statistical evidence. In this situation, you
pretty much have to work from engineering first principles.

Let's start from what is scientifically defensible:

Operating 50 degrees LOP vs 50 degrees ROP (which is what many
manufacturers recommend) means that:
The engine runs slightly rougher. Extra vibration.
The peak pressures in the cylinder (and thus transmitted to the
crankshaft) are lower. Less stress on crankshaft, bearings, etc.

That's about it. Everything else is rumor, conjecture, and guesswork.

The slightly rougher running may in the end reduce the life of the
engine more - or less - than the higher peak pressures in the
cylinders.

Oops, I guess we're done until an actual controlled study shows us
which factor is more important.

Michael