Couple weeks later, so here's the update...
I opened up the case and found the adjuster plunger was out 23mm... I have no idea how it jumped from 17mm to 23mm after going 60 miles, but it looks like it happened. I gave the plunger a little push and the thing popped apart. Crisis averted, I guess.
I ordered a new chain and tensioner (rear guide). After reassembly, the plunger was out 9mm. I got everything buttoned up (broke a head bolt, but I'm leaving it since it doesn't leak). Runs nice and strong now. I'm going to ride it as-is the rest of the season, if possible.
Right now the only issue is that it is backfiring like the dickens. I'm pretty sure all my exhaust connections are tight, but I'll recheck and I still need to do the white spacer mod.
Thanks, everyone, for all your help and patience through this ordeal. To think... it all started because I wanted to shine up a pipe and ended up breaking a header stud. Of course, had I not done that, my cam chain adjuster would've popped apart and I'd be in a worse mess. At least now I definitely have a lot more mechanical confidence! Not bad for a software engineer I guess!