I recommend sticking with vinegar and skipping the hydrochloric unless you have some product specifically diluted for water heaters or are a pro with extensive knowledge on use and safety procedures. The muratic/hydrochloric acid sold for pools is so concentrated that it’s downright dangerous. Need to wear acid gloves, goggles, and be really careful. In an OSHA approved setting pretty sure it would require gown, face shield, and ventilation. Putting in too much would also corrode through the metal inside the water heater very quickly, and if any got in your eyes you could become blind and on skin you would get chemical burns.

Vinegar or citric acid (or even glycolic acid which is in Zepps descaler) are weaker acids and are effective at dissolving minerals but gentle enough on the water heater internals so really good for this application. Still be careful not to get any in your eyes.

As I understand the issue with a loose neutral to the power company, you get power coming in through hot and instead of returning through the neutral connection to the power co, it’ll go though your ground rod next to your house (since ground and neutral are connected as in your breaker box).

Not an electrician, but I think it’s worth checking hot, neutral, and ground from your lighting circuit are all tight in your breaker box (only open if you know what you’re doing, turn off the main breaker, and beware the life wires coming into the breaker are still hot—touch those or anything they may be in contact with and you are toast).

Another thing that may be worth investigating or reading more about is whether your neutral may be loose somewhere between your house and the power company supply. It may not be super obvious bc stuff will still run but go though your ground wire instead of neutral. That can definitely be a cause for flickering.

An easier solution to try may be to use a flexible claw grabber pickup tool if you have one or have an easy way to get one. Will save the mess of cleaning out the p trap and getting everything re-sealed without a leak.

Also see how good the flooring was laid around the flange. I once switched a toilet to one with a slightly smaller form-factor and suddenly had un-grouted tile exposed…don’t think this would be that common but whoever laid the tile maybe ran out of grout or got lazy but didn’t bother grouting what was covered by the original toilet footprint.