adele is plain awesome. lilia is decent, but not really better than claudia and sherry. then again, lilia does have better hp/mr so she'd be better than both under certain circumstances. robert is useless except for trust item drop rate, so nope. although you still need robert in any case, but you'd also need a better healer eventually (that said I've managed to survive mostly on iris+alissa+dorca only)
kojuro is also useless. liddy is arguably useless for the time being. until they patch the UP generation, but otherwise she can still suffice decently as a duelist (better than elaine lol)
just get 50cc50 for 1 silver of every class, so yes you need to level valerie and leeanne to 50cc50.
as for bandit, not really. but I guess eunice seems to be better overall. you might want to level both eventually since there are times you may need 5-6 duelist/ganker in a map.