Matthew,
I must admit that your problem defies logic - sort of - and is not in accord with what is "supposed" to happen. But such is real life :-) It is puzzling that a symmetrical 111mm spindle doesn't work, since that should be what goes with a standard track setup.
Still, if the asymmetrical 115mm spindle is too tight, then it would seem impossible for the 111mm spindle to work. That is, the 115mm spindle should have 2.5 to 3 extra millimeters on the drive side when compared to the 111mm piece. So the 111mm spindle would have to bring the crank even closer to the chainstay.
What was the chainline measurement with the 115mm spindle? If it was actually something like 42 or 42.5mm, I would have to think that your crank arm's tapers have been stretched. That is, a new crank arm with a symmetrical 111mm spindle should provide a chainline of 42mm to 43mm. So I would expect the 115mm spindle to give you a chainline of at least 45mm, perhaps even 46mm.
In fact, using the chainline as a reference point might
be the best approach to understanding what is happening.
Otherwise, you might be in for new surprises when you
finally find a 151 BCD crank arm.
Best regards,
Fred Rednor - Arlington, Virginia (USA)