In order to obtain a LE demo, you need to prove a reason as far as I know, beyond being a dealer first. The letter stating that a LE agency wants to test/evaluate this firearm is simple enough.
I wish we could go back to pre-86 myself, but it isnt going to happen soon.
As far as becoming a dealer, go get your FFL, then get your SOT, its simple and straight forward. You can do it from your home if your local authority does not prohibit a home based retail business.
So if I get the chief of police of an agency to say that they want to demo a rifle that I purchase, I can purchase the rifle and keep it forever?
There's nothing stopping me from getting my FFL, other than the fact that I don't need it for anything other than the possibility of purchasing a demo rifle (other than maybe doing some transfers here and there...)