I'm in the process of making another one myself.
I try to line the tang up as close to center of possible when drilling. If its curved, just try to keep it as far from the side as you can. I normally have to cut them down a little but try to keep around 2" or more to make sure that there is enough to hold it in there.
As for the guard, I had the same problem. I'm not experienced enough to know how to correct it so I made another. I bough some brass barstock and cut my own.
I normally drill the antler with a bit the size of the narrowest width of the tang and then file it out to make it fit. I can make it fit snug that way and I don't have to worry about it twisting when using it.
I also score the tang in a few places with a die grinder or file to give the glue something extra hold on to.
Here's one of the ones I made about a year ago. (this one did not have a Helle Blade)