I got my Mew Z-crystal, but plan to reset my games so I have one of every legendary on both my games. Can I get the Mew Z-crystal again, or will I forever lose it when I reset?
Do NOT reset your game! Seriously don't. When I was getting all the starters by restarting my game and putting the starter in the Bank, I didn't realize I got Mewnium Z and kept restarting until I started my real game. When I didn't get it again I realized it was a one time event. The only way to get it again is by paying for a new subscription of Poke Bank, but THAT only works if the 3DS never had Poke Bank on it before.
Hope my personal experience helped you.