I'll post an update after my visit to the shop later today, in this thread.

Unfortunately I did NOT notice it when installing the rear wheel. I threw on my tubulars for the first time since I'd gotten the bike, yesterday. I'd previously removed/installed the stock rear wheel, using the stock skewer, and it admittedly always felt like more of a struggle than it should have. Unfortunately I didn't even think to check if something was truly wrong - I actually figured I'd shortened my chain a little too much, making it harder to slide the wheel/cassette in there properly.

When I put my tubulars in, though, I used a different skewer. I tightened it well - but I suspect now that the dropout was actually resting on the spring!! Anyway, tightened it up and went for a ride. While going hard up a hill, the rear lifted a bit and the wheel dropped out. I (luckily) came to a stop with my feet on the ground. Called for a pick up.

Anyway, I feel like I've eaten way too much of this thread's time, so I'll post any additional updates in the separate thread, linked above. Thanks again all.