Still no word from Avis to close my case after 7 days, so I took the initiative and called BWI directly. I asked the agent who answered to please have the station manager call me at his convenience, which he did 90 minutes later. sz2628 predicted some of what happened...
...and the rest was symptomatic of travelers tending to return the cars to the proper place but not waiting around for a receipt. Dunno if that also means when they are slammed they might have to drive the car off for the quick turnaround prep before they disposition the returner's bill, could be.

He said my bill wasn't prepared until Monday morning. The recap, which you readers can skip...
...well, 3-6 were completely fictitious, perhaps out of an over-eagerness to please the boss or something. There were two real renters, one out on Wednesday before any of the times noted above but certainly consistent with sz2628's excellent post. And another at a nearby off-airport location out on Sunday.

To readers who might empathize, you can see that there were at least five times where a more efficient system would have realized my reservation (me) could not have had the car. And that a perhaps-too-paranoid person's concern might extend way beyond the dollar amount of the incorrect billing (there's the value of the car, and the potential that an Aaron Hernandez-type might begin to learn to exploit, sorry). The manager expressed these concerns, not me. Also, "very concerning to me" and "this is a horrible mistake". Of course that could very well be deference more than empathy, which I also appreciate.

The charge will be reversed though it will still take a few days, and he offered me a not insubstantial amount of coupons for next time.