Well I did write a whole WIP some years back on how to process antler materials, its kind of defunct now. Apparently the software here on BF dumps any pics when they are over five years old. Kinda frustrating for those of us that try to help others out but there ya are. Maybe the text will help some:
https://www.bladeforums.com/threads/processing-elk-sheds-a-wip.1467821/
Anyhoo to specifics. Yes fill in the pith spots with super glue. Normally folks will say use thin superglue for this but I prefer Gorilla Glue Super Glue and its thicker. Let er set up and sand er smooth. If you can find some I like to use an accelerator on the super glue, then ya don't have to sit around waiting for it to dry. The accelerator also makes the glue less clear when its used which is good but not an absolute in this application. Sometimes ya may have to do another coat to get it completely filled.
As far as dyeing this is what is used, potassium permanganate (PP). Not only does it give antler that desired color that we are looking for, its primary purpose historically was and still is as a disinfectant. Ya can get small bottles of it on that big river outfit from chemical supply places for pretty cheap. Preppers use it as a fire starter but I don't know the process of that. Make up a solution of it. I take warm water and dissolve the PP into it till it won't take anymore. Then stick your elk or any antler (Sambar has all been treated with this too) for about half an hour. Take it out and let it dry on some paper towels and then hit it with a clear coat of some kind. I've been using Watco spray Lacquer and like it. I tend to use a satin but I don't know that it really matters. Then proceed with your knife building:
In your case, I would tape off your metal and using one of those disposable foamy brushes, I would paint the PP on the handle. I'd probably hold it point up and just paint it on. When I've achieved the color I want I'd let it dry and then clear coat it before sanding. Ya'll need to sand all the smooth parts back smooth and thats what produces the contrast like in the pic above. The white has been sanded and the dark left alone because its the bottom of the texture. Should warn ya that when the elk first comes out of the PP or when you are painting it on it will turn purple. Not to worry it will dry to that darkbrown black color we're looking for.
As far as feeding elk or any antler, collectors will usually give it a light coat of mineral oil once or twice a year.
Hope this helps and glad to answer any questions there might be.