I think you are still misunderstanding how electricity works and possibly misunderstood what SR told you. Most likely SR said something like "we can neither confirm nor deny that what you did will/won't cause the problem". Afterall, from their perspective, they don't want to give advice on something that they can't physically see and check for themselves. In other words, a CYA response - which is underdtandable. There's also the issue of a it being many decades old and possibly many previous owners "having their way" with things. Even further, they're likely not going to give out advice on something fuel related to just anyone that emails them - that's a recipe for disaster for the company.
But, back to the first part... it's just basic electricity - properly adding a second ground CAN NOT (impossible!) "all of sudden" introduce +12V.
I don't want to rehash everything that was discussed in the other post and all of the advice you got there, as well. My main point here was simply that it's best to avoid giving out advice (like in post #15 where you said "My 1990 has only a pink sending wire. No ground. I made a jumper ground. That was a big mistake") that is untrue as someone else reading it may think it's true.
