No changes from stock for C4, frag grenades, or rpgs. Not even their models. All pistols are osok depending on range because of the low health associated with hardcore. You can snipe with pistols because that is how they made the game. Pistols can literally fire as far as sniper rifles. The difference is the fov, recoil and the rest. And obviously the scope.
I will test out the defroster health. It should be pretty tough against bullet weapons but not so much against explosives. Full defrost time is four seconds. Default range is same as a standard sentrygun. The slow 'pulses' are just the fx. Just as the minigun fx for the sentry are just for show. The actual beam is invisible, like player defrosts.
As for the emp, other than bumping up the range the answer is that it functions as intended.