I feel like modular is distinct enough to not be considered a form of amputation. An amputee is completely missing some portion of their body, of which they may or may not have an artificial replacement for (which begs the additional question of if an amputee were to receive an organic replacement grafted on, are the still an amputee because they now have all organic parts, but not the original parts?). Someone who is modular still has all of their original body in a functioning capacity, just that not all the parts need to be connected at all times.

Basically, I feel like to be considered an amputee, something needs to be lost, not simply disconnected.

Well said. That's what I think the difference is between them. Being modular is kind of like a "part time" amputation, rather than a permanent one. They can still use the body part and reattach it, but it's still there at the end of the day. With regard to Ally, her leg is gone forever, i.e. a regular amputation.

Since IIRC she cant put her head back without a curse removal, she's awfully close.

She's a rarity. Generally speaking, amputation of the head isn't really a survivable thing apart from magical modularity, lol.