Please accept my apology for the delay in getting back to you on this. You can rest assured that I have not been ignoring the questions, in fact, quite the contrary is true. Not only have I been repeatedly looking at this problem, I actually copied the photos and pasted them in my photo editing program where I enlarged them about ten times to get a closer look. Now let me share what I have concluded and see if it corresponds to what you have found. Fact- The house is only 6 years old so all work should conform to the codes that are currently in force. Fact-The T&P valve drain line, Water heater drip pan & HVAC condensate drain line must either terminate outside the structure or discharge into an approved indirect waste receptor. Fact-the T&P drain line may not discharge into the drip pan and the T&P drain line & drip pan drain line MAY NOT be connected to a common drain line. FACT-The T&P drain line, drip pan drain line & HVAC condensate drain line must run "Full Bore" from the source to the point of discharge. From my observation of the photos- The 1-1/2" line extends out from the wall then turns upwards by means of a 1/4 bend with a 3" x 1-1/2" reducer bushing and a short section of 3" pipe that probably was about 4" to 6" high, which would create a waste receptor for the T&P drain & HVAC condensate All of the joint on that section of the line show clear evidence of PVC cleaner/solvent with purple dye. All of that work was probably originally done during the initial "Stack out phase" of the plumbing. At that time the walls would have still been open and the full length of that run would be open for inspection. Now here is my first dilemma. Code expressly prohibits downsizing a DWV line in the direction of flow and that would have immediately caught the inspectors eye. Once the inspector realized that the short section of 3" pipe was intended as a receptor for the T&P drain, Pan drain & HVAC condensate line he/she would have classified that 3" as a fixture and would have then checked closely to see where the 1-1/2" waste arm was discharging. If the 1-1/2" line was connected to the DWV system the inspector would have insisted on a trap under the receptor. The lack of a trap suggests that the line is running to the nearest exterior wall and discharging outdoors, which would not require a trap. There is an extremely remote possibility that they installed a trap in the wall as you suggest, however code only permits a maximum of 15" horizontal offset from the fixture drain opening to the trap input; and it appears that the distance from the 1/4bend to the wall is very close to that length. Why would they put the trap in the wall if it could be much easier attached under the receptor on the end of the line where the 1/4 bend is? It is possible that with the trap on the end of the line it would have exceeded the maximum allowable length from the trap weir to the vent opening for an 1-1/2" line (see my post on "Fixture arm defined") so they installed a trap in the wall and attached the horizontal offset to extend the line, but you said that the drain line in the adjoining bathroom is close therefore this simply defies logic. Now here is the problem at hand. If that line discharges outside the structure as I suspect you cannot attach your water treatment discharge line to that line. On the other hand, if that line is attached to the DWV system you must first determine if there is a trap in the wall because code requires a trap for all fixtures attached to the DWV system. You may not just arbitrarily attach a trap as you suggest because code expressly prohibits "Double trapping" (having two traps in series). In fact, years ago all structures were required to have a "House Trap" on the main sewer line where it attaches to the house main drain, but those traps are now outlawed under the no double trapping rule. Now for the other discrepancies noted: Examine the connections where the T&P discharge line, HVAC condensate line and the water heater drip pan are attached. Note that there is no evidence of purple dye on the joints. This suggests to me that those lines were installed by someone else after the original waste receptor was installed and the individual who made those connections was not familiar with code requirements. Note that the HVAC drain line is a 3/4" PVC line and they attached a 3/4" tee to accept the discharge end of the T&P drain line. That is a code violation. Properly to combine both the HVAC drain and the T&P discharge line that MUST BE a 1" Tee. Under the top tee there is a 3/4"x1" bushing increasing the line to 1" to receive the 1" discharge line from the drip pan. At minimum that tee must be 1-1/4". It also appears that in the process of connecting the drip pan drain line they had to cut the 3" down flush with the reducer on the 1-1/2" quarter bend therefore you no longer have a receptor. Properly the 1-1/2" line should have been a bit lower, but lacking that, the water heater should have been raised up on bricks or other suitable supports to allow vertical clearance for the drip pan line into the receptor. You also stated that the only line visible outside the structure is a cleanout, which is 30' from the building. Code requires that on slab construction the Main Cleanout must be at the junction of the house main drain and house sewer within 3' of the footer wall. I suspect you main cleanout may be slightly covered with dirt and not visible and the cleanout that you found is at a change in direction of the house sewer. Code requires on a house sewer we must have a cleanout at any change of direction which is 45deg or greater and one for every 80 linear 80' of run. As much as I hate to say it, in my humble opinion the only sure way to resolve the issue here would be to make a small opening in the wall and physically see if the line in question is attached to the DWV system and insure whether or not there is a trap. BUT,there is a metal perf strap securing the line to the wood blocking behind the 1/4bend. Code expressly prohibits using metal hangers on plastic pipe. The copper T&P discharge line is discharging into the top of a 3/4" PVC tee from the HVAC condensate line. Under the Tee there is a 3/4"x1" bushing which is enlarging the line to 1" before it enters the 1" tee from the drip pan.

It is allowed to have bends in "the rest of pipe (the extension) that runs to the drainage pipe". Straight is better but not a code requirement. To say "else the water will not drain well" is a subjective matter, as what does "well" mean, given that bends are allowed. If bends were really serious, they would be disallowed, not permitted, forbidden.

A macerating toilet might be a solution for you, but you will need a plumber to look over your situation and decide which, if any, of the many macerators will work. A macerating toilet flushes into a chamber where waste and (more importantly) toilet paper is ground up into a thick slurry. It is the slurry that is then discharged into a household drain that can be as small as 1.5" with some models. The macerator needs electricity to work, so it must have an electrical source nearby. The principal is similar to using a disposer at your kitchen sink. A chunk of potato would undoubtedly clog your drain, but after the chunk is ground up by the disposer, it passes easily. However, (and I cannot put enough emphasis on this) such an installation is NOT a DIY project. Trust only a master plumber with the installation, and make sure it is in full compliance with your local plumbing code. Some localities do not allow macerators (some don't even allow disposers).

Do you have an electric water heater? Copper plumbing? A fractured heating element in the WH can place significant amounts of voltage on the plumbing system. Or the dishwasher may have a ground fault when it's not operating, or your refrigerator through its icemaker. There are lots of possible sources.. If your plumbing system isn't properly bonded to ground, it can float at whatever voltage is present. It's not too uncommon for the bonding jumper across the water heater to be missing. Now and then the source can even be outside the house. In the early 2000s I ran across a weird situation where a friend's phone line read over 70 volts to ground. It was enough to give me a pretty good tingle. He called the phone company and they wouldn't believe him. He never did get it fixed.