First off, I'm a lot more forgiving of residual pressure valves than I used to be.
That 37 Olds I worked on with a manual single master driving a boosted single master driving 4w drum brakes did it to me.
Effectively the only thing that a residual pressure valve does in a disc system is eliminate any "take up" before the brakes begin to function but you know that, siphon drawdown is one problem but you know all about that too based on your post. ..
If you have a slightly out of true rotor (very common, especially on vehicles driven "spiritedly") and no valve it will knock each side back separately resulting in a wider gap, with a valve in place it will push fluid back and forth between the pistons (or back and forth on the sliders for a single piston) only releasing the pressure above 1/2 psi back to the master.
Hot or cold it shouldn't make a difference on pressure settings.
I'm having a hard time blaming the residual valve for dragging when the brakes are heated unless it's in a location that gets the valve itself hot. .. I suppose it could be because of the friction material itself getting more ornery when it's hot?
Still, 1/2 psi ain't very much applied pressure. ..
I'm interested in how much free play you have in the pedal when the system is heated up compared to when it's cold.