1) It's not structured the same way. A normal plan discounts a percentage off the plan with the line charge separate. Unlimited plans have the service and line charge as one. You get different discounts in the form of automatic discounts based off line count, autopay and first responders / military.
2) You don't lose it, your discount simply won't do anything for the bill. Secondary discounts still apply like for accessories.
3) You can do this at point of purchase only.
I tried Google's Project Fi which used a network made up of Sprint, T-Mobile and WiFi...service was atrocious. It was cheap, but unless you don't mind emergency calls only, I wouldn't bother.