Bathroom Tub Handle Leak
So with both valves off there is no leak from the cold water "handle" nor from the spout, right? This means neither the washers (seals) nor the seats are leaking. Leave them alone.

With the cold water valve closed and the hot opened there is a leak from the "cold water handle", right? This means it is leaking around the stem of the cold valve. Most probably this is a leak around the shaft of the right valve, but could be around the threads of the packing nut. First thing to try is to tighten the packing nut to compress the packing around the shaft.

Actually the first thing is examine the right valve while it is leaking. Take the handle off and the fittings around the valve and examine it with a flashlight while it is leaking. You should be able to tell whether the leak is from the thread of the packing nut or around the shaft. First try tightening the packing nut in steps (but don't overtighten) and see if this stops the leak.

If the leak is at the threads of the packing nut and progressive tightening does not stop the leak, you might try unscrewing the packing the nut and wrapping teflon tape around it (3 turns (layers) of tape) and reinserting and retightening.

EDIT When tightening the packing nut it is important that the valve not be opened so far that the packing nut does not press against the end of the stem. The valve should be almost closed but open enough so that the mixing chamber is filled with water. And when reinserting the stem assembly after it has been removed open the valve all or most of the way so that the seal does not contact the seat while tightening the packing nut.