Mech. fuel pump failure modes?

Good point, conincident events may not allways be related! :)

Well, I re-did the original vapour return line with 8mm ID tubing hooked it up to one of the fuel tank "vapour condenser" inlets, and hooked it up to the regulator bleed-off with a piece of flexible fuel line. At idle before driving off today, I pinched that flexible hose with my fingers, and I could both feel and hear the fuel rushing through the "pinch" so that seems to be flowing quite freely.

The fuel cap is fully vented and there are never any signs of under-or over-pressure in the tank.