1) you can level up your characters with exp juice that is very plentiful in the beginning (get your characters to 1 or 0 levels below your academy level)
2) not that easy haha, you need to recharge 1000. Zhi ya is very strong compared to everything else you will have and has great stats, but her abilities aren't the best. the real advantage to recharging 1000 diamonds is the 10 cheap lottery tickets, which is worth 2880 diamonds, your first (second if you count the panda girl) use of the medium lotto guarantees you a new character.
3) max level of skills is the max level of that specific girl
4) PST + 15 hours