If it is possible, I wouldn't put it past Amycus Carrow to have used the boy's blood to weave a curse that would injure the boy should he either rebel or try to escape. In fact, I'd think it exactly the sort of thing he would do.

What worries me is that he might also think of binding the child with a curse that would kill him if anything at all were ever to befall Carrow himself. Something like your first example (if it could be designed to avoid reciprocity).

Carrow's not the sort to leave his belongings for others to enjoy. As it were. Frankly, I've long wondered if he and Alecto have bound themselves with that sort of death pact. Did you have a chance to observe them together when you were in the castle, Sirius? It's unseemly in the extreme.

Your final point is a good one, though: mixing his blood with the boy's might, in fact, transgress Carrow's personal boundaries.

Or it might not. So much that we find taboo means nothing to him; I can't say with any surety where he draws those lines in his own sensibilities.