So am I understanding right that to do floors 101+ you have to be in a fixed group and stay in that fixed group all the way until your attempt ends? WTF?
The only way to get into 101+ is have a save file clear of floor 100 in a fixed group with no wipes. You then use that file for all 10 sets of challenge floors.
The only way to get that is to start in a fixed group at floor 51. If the group wipes between 51 and 100, you need to start over from 51 to have a 'clean' file to access 101.
You're free to leave after 10 floors like always, so you can tackle all the floors back-to-back or one set of 10 a day or whatever you want, but you can only reenter with the same fixed party you had before.
If you wipe after floor 101, your save is no longer valid and you need to restart the process from floor 51. So basically you're not changing up your group from 51 on unless you wipe and have to restart.