Why hope not? You know I think you're right, it is from Blood Rites
Why? For one, I think (and this is just my opinion) that we need a break from the character we know as Murphy. She becomes a Valkyrie, we see her by the end of the next book. That's no break from her. That doesn't help Harry heal. That puts him right back to where he was, not dealing with anything that has happened in his life. And puts us back to dealing with Harry not wanting her to be killed again, keeping her safe, her acting like she knows 20 times more than him, just her character grinding on some of us through the story. Her character arc needs a revamping and her becoming a Valkyrie just puts her right back where she was, with magic and a better health plan.
Which brings me to my second reason. When I say the arc needs a revamp, I don't mean an upgrade. I just mean a retooling. She was always a ball of stress. But when Harry died, the ball got tighter and tighter until it broke. Maybe it broke when they kissed in SG, maybe when she died. Either way, until it did, she wasn't thinking clearly. Her character (the character and her personality) needs to have a vacation and heal. As Gard said, she has earned her rest.
Which brings me to point three. When you lay out the character types of the series, each of the allies play a role in what makes Harry, Harry. The Knights represent his faith/spirit. Thomas and Maggie are family/blood. The Alphas are strength. Molly is magic. Murphy represented the mind. She was the thinker. She was the one who helped kept Harry grounded in his thoughts, to think things through. Her coming back as anything more than what she was upsets the balance set here. She could start thinking like a magic user instead of the detective that she has always been and is what they need at that time. So unless Molly becomes the new body of mind because of her Lady intellectus, Harry is going to needs Murphy to be it when she comes back for Ragnarock during the BAT.
Oh, and she already turned down the offer once in aftermath to become a Valkyrie. Doesn't feel like something she would go back on, especially seeing how Harry is handling the Knights mantle.