Check out the evolution of my saga with my water pressure and my water heater here: Water Pressure Regulator not regulating after rebuild
Never had any issue with Temperature Pressure Release (TPR) valve. In fact, I never even knew it existed till I was working on the 2nd issue above, and I decided to test it out.
The first time engaging it, the lever was stuck, and I had to force it to open. Once it finally did open, released water out of the water heater like I expected, but after I shut it back off, water kept dripping out of the spout.
Apparently if they havent been operated in a long time, they can get filled up with sediment, and fail to close shut after you release them.
After I watched this YouTube Why is my Water Heater Release Valve Leaking and How to Fix, I thought it would be an easy replacement of the bad TPR, but then I realized the pipes were soldered in, and I didn't know to release them:
So... after a couple trips to Home Depot, I arrived to the solution below in my Answer.