Good questions.
As for the guard, I had it laid out flat but profiled roughly. I then put a center line from point to point and milled and filed the slot, using the enter line as a reference, a little undersized so I could settle it into place with needle files after the points were curved around forward by heating and tapping. I got the slot fitting good and snug and drifted the points to line up with the cutting edges.
The grooves in the ivory. I took a piece of 15N20 band saw material and ground the general shape of the topography into it. I sharpened the edge as best I could then used it to scrape out the groove. The groove follows the outside profile so I used the outside edge of the ivory as a guide. This, of course, meant that it had to be sized already. For the guide fence to ride against the ivory, I rounded it a little and buffed it smooth so it would just slide and not scratch. A little sanding took away any burnishing that it left.
Here's a picture of the tool as I was just testing it. The ivory was too thick at this point so I was getting a feel of how it cut. Also, I knew that I wanted to remove the rough edges of the pin holes, so I was going to remove some of the surface anyway.
The slot in the gold was fitting. I just failed to hold it up tight with my fingers. I just had the guard and the gold slid on loosely. Try to concentrate on too many things at once.