In the first post you said the electric motor battery isn't charged. How low is it, the HV battery has to be charged enough to start the gas engine, the 12V does not do this. So, if it is super low, when you jump start it, the charge is too low to run on battery, but also the battery could be too low to start the engine. You would need to charge the HV battery AND replace the 12V to solve the entire problem.