Yeah, this is cool! I recently taught a longtermism MA course, am currently doing an online fellowship version of the course, and have been reading a good amount of GPI's philosophy stuff, so I might be interested in helping out if you'd find that useful.

That's interesting! The path from the difference principle to extinction (or any sort of existential conclusion) isn't obvious to me, so maybe you can say more? Is the thought that creating more people won't generally benefit the worst off, because some of the new people will have very bad lives?

If so, I still wonder whether the difference principle applies. On its face, the DP forbids socio-economic inequalities that don't benefit the worst-off. But there a few different ways of taking this. 

On a very broad reading, the DP forbids any action which partly causes a state of impermissible inequality, regardless of the nature of that action. (So, having kids who turn out to be poorly off would count.) 

On a narrower reading -- which I guess is the way I'd read it -- the DP is only forbidding something like socio-political systems which predictably lead to impermissible inequalities. It's not a moral principle meant to apply to all actions whatsoever, but a principle of justice meant to apply to laws and institutions.

The narrower reading seems better because, among other reasons, the broad reading threatens to make almost every action impermissible. (A Rawlsian Paralysis Argument!) But maybe none of this is what you had in mind?