Okay - if you've gone after all the 'typical' stuff, it's time to get a
little creative.  First, I'd see about getting a couple of pressure gauges ,
and hook them up.  You can either get a gauge and adapter that will replace
the bleeder screw, or a set of pads that replace the brake pads and get
squeezed by the caliper.  The ones that screw into the bleeder are better,
IMO.  Check for equal brake pressure when the brakes are applied - both with
the engine off and with the engine running.  Also check to make sure both
gauges drop back to zero when the brakes are released.  If they don't check
out, you've obviously got problems upstream - possibly the master cylinder.

If the pressures are equal, then you've got some kind of mechanical binding
happening, either on the left side during apply, or on the right side during
the return.   I'm assuming you've already checked/greased the caliper slide
bolts, but I'd check again.  Maybe remove the brake pads, re-install the
caliper, and make sure you can manually slide it back and forth without
anything hanging up.  Again, check both sides.