Einherjar can’t return until all mortal memory of them as a person has faded. So we won’t be seeing our Murph again.
It's possible that this might change if the BAT includes actual Ragnarok, as I think all the Einherjar are supposed to fight then.
Even if Murphy can't return
to the mortal world that doesn't mean that Harry and Murphy can't fight together, if the battle is fought in the Nevernever. Perhaps the Plains of Vigrid (where Ragnarok is supposed to be fought) is in the NN.
And I’m kinda pissed that it didn’t even take half a book for Harry to have more romantic prospects again. He wasn’t over Susan for ages, and he and Murph were way closer.
Harry certainly isn't seeing it as a "romantic prospect", and he is
this close to outright defying Mab over it, when Molly stops him. He's extremely upset about the idea.