CMAW; I am not expert enough to provide a real answer, but first I would want to strip the paint off fully and take close up pictures to inspect. Be sure to take some looking in through the BB shell to see where the tubes are inserted into the shell (discussed below). Do you know what the value of the frame is or how rare it is in terms of C&V aspects.

My gut feel is that the chainstays and maybe all the tubes may have not have been fitted to the ID of the BB shell which will add a troublesome stress ridge to each joint. Also the shells' lugs for all the inserted tubes may either been too short from the OEM or have been widdled down too much by the builder. Both probably, but the goal was likely to make them look more "artsie". The amount of metal to metal looks minimal all around imho.

Fixing it seem dubious to me. I would recommend poking the question to Frank (FTW). He might see that a good TIG welder could run a line of weld down each of the cracks in the shell and get enough penetration to provide at least a minimally acceptable repair. Although I do braze over weld often, I have never welded over a brazed joint. But I think I have heard others say that the brass can polute a weld. I am thinking that I am thinking TIG as it might avoid throwing a lot of extra heat at the larger joints associated with the BB and thus might not disrupt the brass in those areas or reduce the amount of re-brazing needed after the shell integrity is restored by welding.

Anyway, it 110% fact that whatever Frank says is what you should do. Trust the Force.

/K