About that 100 years later Miyabi with Temma Jr...
Temma-kun is in a loop like Miyabi, he's forever 20...until he becomes a full-fledged knight or something, right
I imagine he prolongs that as long as he can to keep Miyabi (and others) company, but eventually he has to graduate from being 20 years old, after all. That explains how his kid, 100 years later, might not be so old yet (or maybe the same logic applies to his kid - staying in a certain age until they reach a certain point).

A Glimpse of Us OUT-TAKE #1 It’s not rare for Uki to wake in the middle of the night, especially when someone else is having a rough sleep – psychic senses. But sometimes he’d sleep through the quiet warning bells in his head regardless, drawn into a deep sleep, courtesy of prolonged sleep-deprivation, and Uki would find himself being shook awake by another one of Noctyx’s members – usually Yugo, whose room is closest to Fulgur’s.