After reading your puppy's "origin story" (what a CUTIE! btw
) I actually had a new perspective on the situation with your MIL:
Instead of Puppy just being yet another pet you and DH have adopted, Puppy is The Special Valentine Miracle That Was Sent From Above. So maybe this is what is causing MIL to gush so much over this unexpected "bundle of joy." It could be a Hallmark Movie: "The Miracle In Locker 27" or "Puppy Love"...er...too much?
Anyway, my point is maybe MIL is caught up in the romanticizing (no pun intended
) of Puppy's heartwarming story. From her POV (especially based on how you describe her reacting to Puppy), in your family dynamic Puppy is a minor celebrity - like the kid who gets rescued from the well or the guy who bowled a perfect game. Instead of just New she is New and Different and Special. Since MIL doesn't
happen to have any human grand-kids to distract her, she has latched onto Pups shiny specialness.
Hopefully after a little bit of time the bloom with be off Puppy's rose, and MIL will dial back her fangirl reaction to her and just see her as simply another pet (albeit a very cute one!).