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Every major code in North America requires a vent on a sewage sump, they only differ by the size of the vent. Vents for the fixtures draining into the sump don't count.

It's not permitted for potable water by code but some dude with a lab coat will have to tell you if its safe :)

Flat vents (no such thing in the NPC) are really just horizontal vents and there certainly not illegal if installed above the horizontal center line of the soil pipe. A cleanout should be installed...

Zurn does make good quality drains, check out Watts as well.
As for the shape, that's really up to the style you prefer, they will both work just fine.

Precharged tanks wear out, I doubt it has anything to do with your water quality, but if you just installed a new one and it's leaking; even slightly, take it back for a replacement.

Have a look at this article for more info on how a mixing valve works.
Thermostatic mixing valves
Hope that helps.

They are approved but I would never install one. Braided supplies under counters and rigid chrome in sight(if the homeowner wants).

I have one possible explanation, there is a sag in the shower's trap arm possibly caused by cutting a tee into the stack when they did work in your basement. This could cause the trap to be siphoned...

The sizes in the code are the minimum size not the required size;)

Holy cross connection batman, leave an air gap between the supply pipe and the fish tank for one(at least an inch) and google "thermostatic mixing valve" your fish will thank you.:D

Yikes, if the foundation is still good perhaps you build a new home on top?....lol