Flattening horn is possible. Chelsea (Squeaky M) is correct about the heat. Use LOW heat, like no more than boiling water temperature. I have not tried to flatten impala, and always used it the shape it is naturally. But other specie of horns flatten out fine, I see no reason for this to be much different. You can boil it to flatten it, but then drying it back out without cracking can be problematic sometimes. I would suggest dry heat, like the lowest setting on your oven. It may (WILL) smell the place up a bit, be forewarned.

Buffalo horn would be easier to deal with in flats already. Lots of horn buttons are made from the tip cross sections where it is solid.

Don't get it real hot working it with a belt sander (or drum/burr in the dremel). It stinks but it is not particularly hazardous whilst working it.

Antler would be more traditional as a highland button material (I think), but horn is nice too. It (Impala horn) really makes a very nice knife handle, if the curve suits the design of the knife. The ridges make lovely finger grooves in the right application, and what that is, is entirely up to the taste of the maker. Enjoy.