Ukraine: Why the current US spin?

Uh, no.

This is my field (folklore), so I speak with some knowledge, though I study neither of these legends specifically. The organ theft narrative and the blood libel are clearly related, in that they share the basic structure of an insidious them-group kidnapping vulnerable members of the us-group for use solely as body parts, for sacrifice or transplant or whatever. You are correct to say they’re not the same, but you are incorrect to say they’re not related. You’re also throwing the terms “myth” and “mythical” about quite casually in a discussion of traditional narratives, which is unhelpful since these are also technical terms in that field.

When a particular aspect of a legend is discredited, but the need for the legend is still there, the community adjusts or shifts its narrative.

TL; DR: While not the same story, they are absolutely related, and denying one while claiming the other “might be true” is pretty par for the course of a narrative shift.