Some people advocate re-doing new accounts until you get a black rarity unit from your starter rolls, although that's your choice. You did get a platinum princess, which is a somewhat hard-to-get yet necessary unit - so grats on that.

1. I can't comment on your witch since I don't have her ... maybe someone else can tell if she's good enough to clear War of Magic for you.

2. I have Liddy maxed (except CR/SU) and don't regret it, although I find that I don't use her all that often - which I suppose makes her niche. Anyway she's much like a gold valkyrie as a duelist, but can also weaken a boss or take out a whole enemy wave occasionally with her skill. She's useful, but not game-breaking. Since new players don't get a Kerry anymore I'd suggest Liddy as a replacement.

4. I'd recommend you ditch Robert for Alissa, because she's a stronger healer before awakening (and you won't be doing that for months). Robert is mostly useful for male-only missions and getting more trust item drops.

5. I have Kojuro as well, but samurai are very niche, and him only being gold doesn't help. So while I've gotten him close to max level, I only recently found the third map for which he is actually useful to have. I'd advise against using him for your core team.

6. Max out both of them unless you get better replacements before you do. Even then they'll be useful as secondary units of their respective classes.

7. Personally I don't use bandits at all, so I don't think it really matters.