I have rented recently a full 80 mm Hasselblad set and all frames are about few mm narrower than frames from my Yashica TLR. Also all corners are tapered.
The A12 film holder and its insert look OK, nothing unusual.
Is this normal? If not, what causes this effect?
This is normal. Hasselblad negatives/slides are in fact very slightly smaller (maybe 1-2mm) than those that come from a Rollei and maybe Yashica as well. The 4 jags at the edges are pretty normal, they make a Hasselblad neg/slide instantly recognizable.
We just moved, so I have no idea where my negative binder is at the moment to look up my old 'blad negs, but why is the op's negative curved/misshapen like that on the bottom? It seems the film guide would not give that result, judging by the pics of the back that S.G. posted. It looks as if the film strip had lifted away from the film box or something, and it's irregularly curved the same way on the negative below as well.
Those shapes are not from the silver clamp (there's only a clamp on one side and it's much shorter than the side of the frame).
The shapes come from the space between the ends of the formed film rail and the rollers. Pull the dark slide out of an empty back and you will see this.