How can I rewatch all of the séances I already unlocked

That feature isn't available yet but I do have plans to add it in a future update!

yes it is, theres a red orb in the MC's bedroom, that gives you a choice of three places

Bust room, pixel room and scene replay room

Scene replay room is self explanitory, bust room lets you see specific characters posing in specific moods, and pixel room lets you replay any animation that happens between you and a pixilated mob like the naga just before you fight medusa

Just a disclaimer, im not here to be an A-hole, i swear, just wanted to add helpful input that i found through my exploration of the game 

Odd, I am not finding any red orb in the MCs bedroom.