The worst aspect of my experience was that by the time I knew for certain it was a scam (certain things he told me raised red flags, but didn't trigger a full refusal to continue), it had already costed me fifteen short of $300.

At that point and doing research following a bigger loss on top of that, I engaged in sunk cost action and simply told him "keep me updated on whether you can pull all that out to your Lightning wallet or not."

I was pissed; at both his falling for it, and myself being the biggest loser.

Damn. As a younger brother (I'm 33 he's 40ish) i cringed at how shamed I would feel for misleading my bro due to my own.. Whatever it was. Blindness, stupidity, naivety.
I'd be worried that you'd never trust me again.