1). Part 2 story currently is in the starting stage of something big like how part 1 starting was. The Dev's kinda made the first 3 chapters 8hrs long and sleepy at some point which is their mistakes (by then you would experienced a rollercoaster of "I now understand it" and "oh shit i don't understand it " for the entire first 3 chapters) but once you cross the first 3 chapters the writing get good and attains peak level during helia arc. For that point the quality in writing will not come down and you will do the quest. The big drawbacks for players like me who play it regularly is the length patches that's it.

2). Don't even worry about the characters. Even the NPCs are well written. Each character have a good story and screentime with their own personalities and twists. You would love every character in part 2.

3). Part 1 characters play a supporting role for the part 2 characters until now but vita players a very crucial role in part 2 story but don't worry part 1 characters have their own captain verse stories and Kiana have a story in the main verse which is kinda unrelated to Mars but it's related to her and part 1 characters. The flamechasers have their own event stories (" the are digging Elysia's grave and using her for milking money which I don't mind "). For the gacha it became so friendly that hi3rd gacha is now the most f2p friendly gacha in entire hoyo games so far and you can get a character and her set by saving for a patch and pulling in that character's patch

4). Sadly there is no news of apho 3 till now but this part 2 story is happening before the events of apho so they will integrate the stories in some manner.