Well, I can explain a lot of the scratched heads you faced, you have to understand something right away. Well a few things really. First off, most machinists disconnect any mechinical interaction when you talk to them about something needing to be machined. They are almost all exceptionally mechanically inclined, in that they can build, disassemble, re-create, and design anything. It is just that you have to think about things in a different manner when you are going to cut a solid object into something. So the easy way is to keep the mind set apart and think either about how a part needs to be cut OR how a part interacts in a machine, its hard to do both at the same time without drawing it.
Over that hurdle, also keep in mind that most folks, even ones that shoot a lot, don't know how a bullet is made, and many do not understand the various functions of bullet design. So when you say to them a bullet mould they are lost. That and, while I'm sure most sales folks know the products they have and intended use, they also require you to know what you are asking about as well, or at least to ask the right questions to get the right answers. There is just too many cutters are various ways to use them for a sales guy to know it all.
On the cutter, yes, grind away the material you will not need, now you can go about it two ways, you can cut it all out and leave just one .090 x .452 or you can cut it in a array, where you can cut all the groves in one shot. Just depends on how much grinding you want to do. Two things need to be mentioned here, #1 when you grind the reamer, be careful not to get it too hot or you will ruin the temper of the steel and it will not cut well for long, so grind a little and cool it in oil, grind, cool etc. If you get it red hot you've gone too far, the fins on a reamer will heat quickly (and become dull), so keep it as cool as you can. Also, you will need to square the front of it, when you get it it will have about a 45 degree chamfer on the nose, you'll want to gind that flat to a 90 for good clean driving bands, same thing on the backside after you grind the metal you do not want away.