Neutral 14 ga wires hot
Larry, a white wire can be used as a hot and if so is supposed to be marked with black tape or otherwise marked black. You are calling it a neutral, but, if I understand your explanation, when you connect it to true neutrals and turn on the breaker it immediately trips. This means it is a hot.

Stop connecting it to neutrals. The black marked white must be feeding something. This is what I think, but I am not an experienced electrician and have never worked on anything but my own housen and it is wired in NM. Unless someone here can help you, I fully agree that it is time for you to call an electrician.

The only thing in my limited knowledge that I can imagine is that this black marked white is or was connected to a switch. This switch controlled a receptacle or half of the duplex receptacle. If the latter, the tab would have been broken and the other half powered directly. Do you have a switch near this box which seemingly doesn't do anything? Is it on? Even if the switch is off it may have been disconnected and the two wires connected together and the switch replaced. This would be contrary to code, but could have been done.

Does everything work if you just cap this black marked white wire? If so, just leave it capped and accept that you have an abandoned wire in the wall. If my suggestion is right, then one of the black wires goes to the same switch and it could also be disconnected with no loss of function.