1. Yes losen the clamp. Yes there's more. Look further back on the back side of the muffler, there should be a bracket with two bolts. I'd remove the clamp BEFORE you remove those two bolts, just so the clamp/rest of the exhaust isn't what's holding up the muffler. I'd pick up a new exhaust gasket from your local friendly kawasaki stealership. Make sure you tell them it's to connect the muffler to the H pipe. The gaskets run about $15 each, you'll need two. As I'm a cheap steamwhistle, I only have half the gasket in mine, and it was a pain in the "O Ring". Though I DID accidently destroy it, not thinking that I'd need it again.
2/3. I don't know about removing the core of the muffler, the parts diagram on bikebandit doesn't show anything removable from it. It probably requires cutting and welding. A FSM should show you the innards of the muffler though (at least the 650R FSM does). Though My guess is that you could take a length of re-bar, or any other long metal pole that'll fit in the exhaust hole, and hammer it through anything that's in there. This is the cheap way. The right way is to just buy a new exhaust for it. Those stock cans will run you about $140 each if you "fudge" them up and want to replace them. Though, fwiw, if it were my bike, I'd rebar the "poo poo" out of the baffles in them
. It WILL get louder everywhere in the rev range.
Note: Information specific to your bike was gleaned from the parts fisk at www.bikebandit.com
Just click OEM parts and go through the menus to get to your bike. It doesn't give specific instructions, but gives you an idea as to what you need to do to get some stuff apart.