According to the marking on the tube it's rated for both EN14680 (Adhesives for non-pressure thermoplastics piping systems) as well as EN14814 (Adhesives for thermoplastic piping systems for fluids under pressure).
The product datasheet I found says:
Tangit PVC-U is suitable for producing tensile stress resistant
connections between pipes (e.g. drinking ...
My gas line can't be pressurized since there is a pressure regulator installed after the gas shutoff valve.
That's... not how it works. Regulators don't vent in an over-pressure situation, they just close. If it's venting, then you have a bad regulator.
You should have tested with your regulator in position. Your pressure test did not test the regulator ...
There are too many variables involved; it isn't really feasible to give a specific answer. You can determine an answer for your situation by determining the main supply line pipe material and length, checking for excessive quantities of elbows (ie more than just a couple), and then consulting a pressure loss table. For example, The Engineering Toolbox web ...
If the distance is less than 5-6’ horizontally from the vent to the new drain normally a single vent will be enough. I have seen contractors double this length on 4” drain lines when the new 2” line ties into a 4” drain
We got an engineer in to deal with issues outside of the scope of the question, and Viessmann support would talk to him, they pointed out that it wasn't a test mode or crash, it was the LCD brightness! Hold down the OK button, and press the minus button (-) (not originally pictured, below the plus (+)) and turn down the brightness, fixed!
what is the best means to connect a new PVC p-trap to the existing galvanized stub out?
The plan you are proposing is best (with some things to consider, see below).
...Would ...a brass waste connector ...be better?
YES, for the reasons you (and the manufacturer) state.
...pipe dope or teflon tape ...?
I prefer tape because it is cleaner, and dope ...
It looks like you have 1.5" pipe reducing to 1.25" (1 1/4"). The easiest way to handle this is with a reducing coupler (example).
This should be less hassle than the reducing ring (no messing with gunky threads) and you should get a better seal overall
That supply line should be disconnected at the shutoff valve as well and should then be fairly easy to bend and remove. The plastic nut at the base of the tank can then be removed to change out the fill valve.
You'll want to replace that supply line with a flexible line just a little longer than the distance from the shutoff valve to the base of the tank. If ...
I use dental floss , it will cut through just about any silicone adhesive/ sealant. Slip it in the crack and pull back and forth. I do this to remove equipment plaques on machines when updates are needed.