jixxerbill and country,
Firtst thing - this is an open forum so please join in the discussion - that goes for anyone else. I appologize that this may have turned into a discussion between a couple of us. Please feel free to chime in anytime you wish.
Second thing, I may have some experience and a lot of opinions but I am no expert. This is a hobby for me and anything I can do to help out others persuing the same is what this is all about.
Jixxerbill, welcome. In reference to your post, D2 will work for swaging dies. When I did my research, I read that O-1 might be a little more abrasion resistant and holds it's diminision a little better than D2 so I decided to use O-1. My experience between D2 and O-1 is that they machine about the same.
In reference to "D" reamer material, there appears to be two ways of you can go. The first is to use drill rod in the annealed state, machine the "D" reamer and then heat treat to Rockwell 60 or above. The second method that I use is to buy drill/reamer blanks which are already hardened above rockwell 60 and grind them to shape. The choice is up to you as both will work.
In reference to lapping, there are a number of materials that can be used for lapps. Brass, aluminum, lead, copper etc. I found Brass and lead to be too soft myself. Aluminum (6061) seems to work pretty well and copper, I found to be great for finishing. It seems that you will have to play around with this and find out what you like.
In reference to multiple heat treatments of the die, I don't think re-heat trating is either called for or desired. I heat treat once then do all my final lapping. If you do your heat trating correctly, the die will be hardened throughout. By the way, I heat treat to about Rockwell 62 then anneal back to Rockwell 50 by imersing the die in molten lead for about an hour. This makes the die less brittle and less prone to fracture (not that I have had a problem with cracking dies). In terms os re-heat treating, it is possible that the die may change dimensions during the heat treat and if this happens in the wrong way, you may be in trouble.
My primary interest is in rifle dies (22, 243, 7mm, 30cal). I would think that pistol swaging dies would be a little easier to make in that they are not as small nor as deep.
If I can help in any way, let me/us know.
Martin