No ideas at this time-- older dogs tend to lose weight,mostly muscle mass but if she's got a great appetite it does seem odd that her weight is going down too. Glad you're taking her in, no one ever knows what could be lurking and the earlier we get on top of it the better for us and our furry friends.
Could Myndi have a case of Alopecia like some older people get? Nothing serious, just hair loss, probably from hormonal changes or maybe from the stress of losing Sparky?

In any case, keeping my fingers crossed and hope you'll have good news for us after the vet check--- good luck and hugs to Myndi