Huh, this is a change. All nightmares who came back previously were those who didn't get sealed properly (Merry fought Ichima before she learned gate sealing, and she had no reason to banish Clione). But this duo got the final "don't come back" sendoff. I guess that's no longer valid and anyone...
@GaoMuscle
Ichima and Clione could return because they were sent away but not sealed -- Ichima because Merry didn't know sealing back then, and Clione because she's a good girl. Those four you mentioned were sealed and shouldn't be able to return.
However, there is one more guy who can return...
I use the reader in long strip mode and "resize to container" setting. I click pages to move to next page. I get to a wide page, and most of the time clicking starts misbehaving on next page and afterwards. Same applies to right arrow keyboard key and ">" button at the page counter.
Trying to go...