I admit I don't know a great deal about horns, but have you thought of trying to work around what happened? For example, you said you wanted to put a lot of fancy metalwork on this horn. True, you said "silver", but what about pewter? Would it be possible to pour some kind of pewter inlay/band around that portion of the horn? It might not be exactly what you had in mind for this horn, but you can always do another one with silver - and by using pewter in a few places on this one you might be able to salvage a lot of work and a nice horn. You could pour a fairly thick band and then shape it either to the octagon, or round, or perhaps use it as a "transitional" band between the two sections?
Just a thought. Like I said, I am certanly not very knowledgable on these things. I do think you can pour pewter on a horn though. Seems to me I saw that done somewhere.
Bounce it off Randy or Scott or Tim or one of the other horn experts. They would know what you could get away with as regards pewter.
As far as another black bison horn goes, you might try contacting TOW.
Good luck with it.