And all that being said, the manga isn't done being written yet, and I think it's clear (and feel free to I-told-you-so me in five years or w/e if this isn't the case) that they're building up to the reveal that Frieren is wrong, the Demons can be reformed.

Yes I do hope so. Too much of the show is too wholesome for a whole lessons of time thing to not be extended to this too.

Most of the show is demonstrating how past ideas can change and improve. It would be a significant blindspot not to include it.

Eventually I suspect it'll be a "Flamme worked this out a thousand years ago but knew nobody was ready for it so she planted the seed to bring about that change when someone would finally be ready"

That or it'll be a "Actually people a thousand years ago had pretty shitty ideas about the future" reveal, but I think Flamme has been built up too much as a prescient genius among geniuses of her time to then go and shit on her later. It's certainly one way it could be taken though.