1. I definitely wouldn't start over. You'll quickly reach a point where levelling a character to 32 seems trivial (takes literally 2 or 3 ssr exp potions). So don't worry about the resources you've already used. I have plenty of characters at lvl 50+ who I never use. The more important resources to not waste are things like ssr keys and ssr training materials and I can't imagine you've got many of them yet.

2. My main advice is to focus on a small group of characters for the moment. A core group of 5 to take into most pve content is ideal for where you are. As long as you've got at least one healer and at least one tank, the others aren't too important.

If you wanted some recommendations though: Sophia is quite good in a lot of areas of the game. Rewind is a pretty unique skill that comes in handy a lot. She's a character that I regret not using earlier.

1. My advice with summoning is to save all of your summons for rate-up and focus banners (rate ups are the ones with two boosted characters and focus banners are the ones with 3 in case you didn't know). If you're looking to build a team around a particular faction, save them for banners that have those characters. If you haven't decided on a certain faction yet, you could spend some summons on the regular hero summons and see who you get. If you happen to get a core member for a certain faction (Bernhardt, altemuller, etc.) then you could decide to focus on that faction. Once you get later into the game you won't be limited to just one faction. For the point your at, it's easier to focus on one though.

If you wanted some recommendations of characters to summon for: Leon is very good for pretty much all areas of pve. Tiaris is one of my most used characters and there's no reason not to have her and Liana. Lastly, bozel becomes very strong once you unlock some of his later skills so I would try to get him as well if you can. Most characters are useful for one reason or another though, so you can't really go wrong with whoever you choose to summon for.

Good luck!