Sounds like you went above and beyond the call of duty with sanitation, water temps. Has your source of CO2 been the same? I have never heard of anyone getting a bad tank. Mine have come from CO2 dealers and fire equipment shops. Never noticed any difference.
Is there something wrong with your tap water other than the possibility of chlorine or chloramines? I went through getting our water report, running it through Brunwater then had to do a very slight acid adjustment. So little I don't bother since I could not tell the difference. One disclaimer though. I'm not that good at detecting small off flavors or describing any I can.
If you think the problem was from the carbonation process, try priming the keg. It's a PIA when you have a perfectly good way to to carbonate without the priming process that is not always perfect.
It might sound stupid but I would try using your tap water and add a campden tablet or less depending on batch size. Just clean and sanitize the keg. I have some that have not been disassembled for years. I half fill them with One Step for a few minutes,run some out the dip tube and turn it over for a few more minutes. Sounds like you are reading too much into it. This is from a guy that REALLY doesn't worry. I have even just rinsed out a recently emptied keg and filled it back up.