Disclaimer: I haven't fully read wtf happened in IS 3 yet lol so I may mistake things here and there. Just brainrot.

Just like how Thorns knows he will use the potion to turn him into seaborn one day, he also knows, in the back of his mind, that the poison he created will definitely be used one day by the requester. Not to harm others, but to commit suicide.

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).