Following up, I had an additional 2,000 base points credit over the weekend, along with another 1,000 for the 50% Diamond Select bonus.
Oddly, I was alerted to check this by an email telling me I'd been awarded 500 points for my stay. Eh?

Got the same email again this morning.
I have not contacted BW again since I started this thread. Thus I can only guess that someone at BW was following this thread, and decided to quietly give me points for the portion of the payment that were my gift cards, but still declined the $100 paid with the card in my brother's name (even though I reimbursed him). Seems odd....