Ok, so I've had a bit of time messing with it this evening. The clutch cable definitely needed to be lubricated, which made setting the freeplay a lot easier. I now have 2mm (as measured with calipers), consistent between the pulled cable sheath or the clutch lever hinge.
However, this hasn't resolved the main problem. If I attempt to use the clutch to shift between first and second anywhere above 3k rpms, it simply doesn't want to go into second and will go into neutral after making a ratcheting sound. If I pull the clutch again (after the revs have dropped more) and try to shift, it pops right in. If I just float-shift by closing the throttle, it shifts just fine. All other up and down shifts are also quite smooth, and it's never jumped out of gear once it's in.
Could the problem be my technique? I spent the last twelve years riding a Honda Shadow with a heel-toe shifter, and honestly used that nearly all the time. I'm still closing the throttle while pulling the clutch with a little preload on the shifter, then pulling it up sharply, but I wonder if I'm missing a step somehow? I don't know why shifting between all the other gears works perfectly but I always get caught in neutral going up from first.
I've wanted one of these bikes since they first came out, and it's really driving me crazy to have such a major issue on day two.