1) If you manage to get all UR, I doubt that the gem cost of additional place is a problem
2) Since plenty of characters are 1* or 2*, you can also get their scene and ditch them afterward.
Anyway, got a lot of luck. Tried the game a bit, I did 4 single rolls (I planned to reroll for a bit, so didn't wait for the 10+1), 2*, then 2*, then 3*, then the fucking lucky UR roll, Krone. She is carrying me so hard. Heavy armor with a godly skill (atk speed x2, crit rate up, become tanky as well). Can solo most of missions.
I can finally fully clear the 3rd event mission, rolling a solid 3,6K/15 stamina. At this rate, i'm done with the event Saturday evening