I know your frustration, but it is going to take some time and patience. Female dogs do mark, so that is not a myth. I am sure the arrival of the new pup has made Marley display certain behaviors. I think some dogs need to just be "the only One'
I am not sure if Marley is or not. If you really feel like things are out of hand and over your head then can you give the baby back ? If not then the old word TIME is what it is going to take. They need time to get to know one another and become friends. I hope it all works out and I hope they become best friends. I am sure with time, patience and your consistency things will be ok.
Good Luck
