I haven't gotten to the end game yet, so I don't know how the Circus Trio ends up, but I would not call their relationship toxic in any way. It's just banter between friends.
If you want to cheese it easily, and don't have the Murkrow, catch 3 Sturdy Gravelers. Buldoze 3 times will cause the cave to collapse, KOing all active pokemon
Looks like this hasn't been answered yet:
There is an entrance to the Underroot from right next to the library.
That's the one that leads to that little section where the megastone is.