It is a puzzler. It sounds like you can reproduce it on a lift or with the wheel jacked up?
I'll apologize in advance if I say things you already know.....
With disk brakes there is no return spring so the pads don't retract, they just stop exerting force against the rotor. As a result, some initial drag is normal after the brake is released.
Because pad movement is miniscule when the brake is released, I wouldn't expect any quantity of fluid to be released by cracking the brake line fitting. However, if there is back pressure, the brake would then release. Opening the bleed screw is probably the easiest way to conduct the test - then you can be sure you are releasing pressure right at the caliper.
If this is what you've already done, then I'm scratching my head too. It's hard to understand why the caliper would be binding up, yet you are still able to manually retract the pads/piston. And just to confirm, are you able to move each piston in without the other moving out? If they only move in equal and opposite directions it might be another indication of a fluid restriction. You'll have to hold the opposing piston somehow, but then you should be able to retract each on it's own.
The temptation is certainly there to replace/build the caliper, but personally I hate doing expensive things like that without clear evidence it's the problem. Almost always when I have conflicting evidence about the source of a problem, it turns out that it's not the problem. It always makes sense when you look back at it - it's that darn foresight that's the problem.
As for the proportioning valve, I have to say I'm drawing a blank on that one. Perhaps a different person made the contributions you are thinking about? I know very little about them, so it's unlikely I provided any real insight.