>>720684609
>It is set after the war with Nemesis
Even after the war, some crests like the one Cai has wasn't spread around until way later.
>She was an adult when she died
Yes, and then she reverted to a child due to being in a weakened state. If enough time passes, there's no reason for her not to grow back into her adult form again in whatever pocket dimension within Byleth (or more likely, some other character that inherited the Crest of Flames) she lives in. However, if the relics are around then she can't be alive.
>Sylvain is irrelevant and lost tech existed in 3 houses
Doesn't change the fact that he made the blueprint for the guns we're seeing in this game.
>Yes, because it is set in the past
You play on casual mode and are wrong. It's a future game.