That's grease. There must be a kitchen sink draining into it.
If that is a lead drum trap I wouldn't start whacking it.
The check valve should be closer to pump.
That pitcher is good other then the DW hose should loop up as high as possible
You have the spring and washer in backwards. The spring goes into the valve body big side first and the washer goes over the spring.
If you called Delta they would have shipped you a new cartridge free of charge.
Turn the water off to the shower valve and unscrew the large nut behind the cartridge and pull the cartridge straight out.
Are le sure that it's not a pex male on the end of the pipe screwed into a female ell and he is just using the pex pipe as a test nipple.
That is one tight bathroom. Code says min. space for a toilet is 30". Plus a elongated toilet will be 2" longer and it may interfere with the door.
You may not make a cross connection but can you say future owners of the property won't?
Sounds like there is no baffle in the tank as in Frodo's drawing.
Before trying to remove the nut turn the stem to half open.
DWV 1/4 bends are not true 90's to allow for the pitch.
post a picture
Tighten it more using two wrenches. One to hold back on the valve the other to tighten the nut.
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