Smoke detector wiring - black wire in 3 wire appears to be damaged. Can I use power from AFCI circuit to power but use red wire to interconnect?
Per:

It seems like the black wire must have been damaged in the wall.

Well, do you honestly feel comfortable knowing there is an exposed hot wire in your wall?
I understand that if you proceed with your plan then both ends will be capped off but will the next owner of your home understand the necessity for turning off two breakers in order to de-power all of the smoke alarms?
I don't think there is any requirement per se for the circuit to be dedicated. Just make sure a regular light switch does not control them per https://homesteady.com/12359235/what-are-buss-fuses-used-for:

The power circuit providing electricity can be a dedicated circuit, which means it is the only device powered by cables that run directly back to the circuit box. It can also be powered by a circuit that serves outlets and lights if the circuit is not controlled by a switch.

When in doubt, check with your local code officer!