The basement bathroom has an exhaust fan that runs between the joists and exits on an outside wall,.The wicked high winds and some vermin, compromised the insulation around the flexi duct tubing, and it was letting extreme cold air in, froze up the pipes crossing the two joists to the shower. iI cut the drywall and removed all the old insulation, temprorarily plugged the hole with a kids football and packed it with insulation .Now it isn't pouring cold air and snow in,.I un thawed the pipes and the water is flowing just fine both hot and cold feeds with no leaks, was a bit dramatic with all the pipe hammering and shuddering at first but its quiet now. So panic is over.
The water looks like it travels over to the shower and then it seems to come back in the opposite direction and then go down the wall to the sink,.
I now have a leaking american standard single handle shower faucet in the bath tub, on the hot side only.
So i shut the hot water feed off, leak stopped, toilet and sink have cold water just fine.
If you turn on the cold water in the bathtub by turning the handle all the way to the cold side,very little water flows out BUT if you turn on the sink single handle faucet to the cold side at the same time,. both taps have full flow. Shut the sink off, and the bathtub slows to a trickle,....
Fair enough, it's all above my pay grade,. i'll fix the bathtub faucet, Hell i can't even figure how to get the handle off,.
I would wait for things to thaw a bit more before I tackled the valve.
I wonder if the water is passing through the valve body. I think that may be an option on some two handle tub/shower faucets. I've never plumbed on that way, but it seems some have that option.
Can you post a picture of the valve, 800 pixels or less?
Where pipes run by air ducts that allow cold air movement, I've seen pipes freeze before.