Some chucklehead might have stuck the bolts up through solid holes, before the flange was glued on.
Instead of mounting the flange to line up with the slots in the right place.
Basically saying, “F the next guy who has to work on this”.
A picture of the actual flange you need advice on...
If you removed that screw while the house water was on, the whole cartridge probably moved forward slightly inside the valve body, and now o rings or seals are not lining up, or have gotten pinched or damaged.
You should turn off the house main water valve, release the pressure by opening some...
The purple stuff is just a stain from the primer for the pvc glue.
It looks like your bolts should come out, there seems to be either dirty old wax on there, or maybe some excess tile adhesive or grout.
Just scrape off the junk as frodo advised.
Then the bolts can slide along that curved...
“A globe valve pointed in the right direction will chatter AND WILL NOT shut off”
I think you meant to say “wrong direction”.
By asking if the valve arrow points towards the water heater, which is downstream of the incoming flow, it is an easier question to answer, to...
There might be a separate pressure balancing module behind the cartridge.
Or you bought the wrong cartridge.
Post a pic of the old cartridge if you still have it.
Pros on here might know what your problem is.
The shower valve setup should not be causing your hot water to run out.
Maybe the cold inlet dip tube in the water heater is rotted away.
That is a fairly common problem on old heaters.
You can find your manual online, to find where the dip tube is accessed for replacement.
I think it is...