Anyway, the original builder said I had worn out the chain (less than 3 months, 700 miles, no cross-chaining). How? "You must be a pedal masher," he said... Could the b-screw be (that) caused the chain to last
only 700 miles?
Chain worn out: Was the chain actually measured with a ruler?
Pedal masher: You don't say what your normal pedaling rate is or what kind of terrain and gearing you have, all of which can have a marked effect on chain life, though something extreme would have to be involved for a chain to be "worn out" in 700 miles.
B-tension: Only if you rode all the time in low gear and the chain was too short, and you only rode up steep hills, and the b-tension screw was really long - and I can't conceive of how the upper pulley could be 4 inches from the rear cog.