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).
Also, what if Temma-kun actually dies not out of old age, but in a battle or an accident and "please take care of my child" is his dying wish to Miyabi...

I also thought of: what if Temma-kun really died in a battle or an accident, so Astel developed a time machine to go back in time to save him and dragged Miyabi along with him, but it took like 100++ years so they all got estranged for a looong time. Maybe Rikka is involved too, as long as he's still around and not scrapped* for some reason.
Cue "his death is inevitable, it's fate" and they can't change it no matter how many times they tried trope and they fall into despair (especially if they have regrets over it, thinking it's their fault and such)...

Pain peko

*I'm also thinking of Vivy: Fluorite Eye's Song anime but with Rikka as Diva/Vivy