John I know you like to try the unusual, and often succeed despite the potential difficulties. And in any case, learn quite a bit in the process -- as do we while watching. So I'm going to say go for it, if it appeals to you.
There is no guarantee that the scroll will match the teeth and slide groove on the3 jaws afterward, true, and the hardening will be lost, also true.
But, there is also no guarantee that the working surfaces will be substantially altered after, no one knows for sure since it hasn't been triedby anyone here -- these are all guesses.
And parts can be hardened and tempered again. So there are possible complications and some risk, but it's up to you if you want to take them on.
If it was me, and I was inclined to try, I'd either try the anneal, saw off, bolt on method if there was enough meat for the bolt on jaws (and I think change jaws would actually be an advantage over fixed), or if not, anneal, saw off, and braze on new (rather than weld). I'd have first covered the teeth and slot with bar soap to reduce scale from the flame. After brazing I'd re-heat and plunge in oil for hardening, and then temper. The worst scenario would be cracking when hardening, which is why I'd try oil first, over water. If it failed to harden, I'd then try water.
Good luck, and looking forward to a blow-by-blow if you do, and no blame if you don't, too!
ps, it will be tricky maintaining the braze while heating to hardening temp -- silver braze may be too low melting temp, so maybe bronze braze, and maybe even plunge harden immediately after the braze flows and solidifies. Or, localized heating for hardening only the teeth and jaw surfaces might be an option -- you could maybe stay away from the braze joint with a small flame.