Mountain Lilly seems to be strong enough, thanks. Maybe i should do her quest, but later - i have too much pierce girls for now to level up. And for Canna - she has not so good stats, but not so bad abilities and not as ugly as for example Fatcia, Sandersonia or Calla and is blunt (i have only 1 blunt from 17 5* girls now) so i couldn't find better option among permanent event girls (Edelweiss has better stats but terrible abilities).
About changing assistant - i pressed this button, it becomes gray, no confirmation window, message "assistant leader" near her model revealed, but nothing changed - girls at main screen and search still random.