Cow horn is a wonderful material to work with, but only the pointy end is solid.
Easiest way to see how much solid you have to work with is to stick a length of wire up the hole, mark it, and then run it up the same path on the outside. Where the wire stops is where you're solid part starts. I'd expect less than four inches with a strong taper.
Still, it's a great piece for spacers and bolsters.
The wall of the horn can be boiled and flattened without any problems. It stinks to high heavens, so do it outside. Do not borrow one of the wife's pots for this job if you value your life. Do it on a windy day.
Just boil the whole thing, bone core and all, and wait. After awhile, and it depends on the horn, the core will separate.
Did I mention that this is a very stinky operation? Remember that.
With the core removed, you can cut the horn into sections. The horn will be very soft when it comes out of the hot water. Just take a section and clamp it between two pieces of wood to cool. If it doesn't come out perfectly flat, just heat it up again and re-clamp.
When hot, you can bend the horn around forms, but expect it to relax a bit once you take it off of the form. Might take two or three heats...
No need for stabilizing. I've never heard of horn being stabilized, but it can be split if you aren't careful with the rivets.