I'm pretty sure that they are loading before the gates open. If my memory isn't entirely gone, I recall watching them load luggage while waiting at the gate for the cleaners to finish.

I've never heard of this happening, and I'm skeptical. I fly a decent amount and my luggage gets misplaced likely in the low percents (more than 1%, less than 5%). I struggle to believe they'll be aware enough to unload a full cargo compartment.

But I could be wrong. Maybe it's just rare enough that I haven't run into it. I would expect it would delay the flight by a noticeable amount. And it would be fairly obvious that they're unloading luggage and then reloading it. It's not like they can just send someone in to climb around and find it. They pack them in tight.