I'm feeling pretty good about it, but keep in mind I haven't actually looked at the bike myself. I'm just another rider and not a mechanic that has seen it all, so take my guess as just that.
You might want to check for any metal fragments in the oil. If you had a catastrophic clutch plate failure there should be some glittering particles in it. Normally they will just start slipping for a while as they wear down, but the sudden loss of all engagement causes me to think that a plate has completely broken up. The pressure plate used in the larger Harleys has earned the nickname "grenade plate" just because of this type of failure.
A "typical" life span for a clutch is from 33,000 km (~20,000 miles) to 95,000 km (~60,000 miles). There is no good average for this because it all depends on how the bike is ridden and maintained. The amount of time the bike is ridden in the friction zone of the clutch is what matters. My daily commute can put less wear on the clutch than a long ride on which I get the bike up to highway speed and burn off a full tank of fuel in sixth gear.
Is it possible that the transmission is shifting fine but the front pulley bolt has broken? Normally that's a pretty spectacular failure, so I doubt you would miss that.
I have the same question about the belt. Does it have anything near proper tension or is it just loosely laying in place with a break in it?