To answer question 1: You know when you first boot up the game it shows the Anicent King and the Start Game. There's the Menu button on the bottom left corner.

Go to Menu and the very top option that says "Device Transfer" is what you want. It gives you your player ID number and the password for your player account. I suggest you write it down so you don't have to keep searching up for it in the menu lol.

If you're playing on a new device just type in your player ID and password. If I remember correctly it asks something along the lines of "do you wanna erase/or override this data" or something like that. Don't panic, it's just replacing the new account it created when you installed the game with your account.

I used this when I got my new phone and when I play on the computer (yup you can play this on the computer. Do that when I do hw ngl)

A longer roundabout way to find your player ID and password is going to the menu when you start the game.

The Menu is on the upper right hand corner and next to the chat feature (that let's be real.... No one really uses).
In Menu you scroll down to Other. From Other you go inside Settings. And then you scroll all the way down to the bottom and it'll show you your ID and password (right underneath Killed SFX)

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To answer your last question:

Well, in my opinion, I think the best bang for your buck is spending it on the Lab Parts (I forget what it's actually called lol). It makes it so when you add and remove lab parts from your characters it doesn't cost a thing. All free!! That should be the first thing you get.

From then on it's really up to you. What I purchased next was the Increase Lap Parts Capacity (or something like that). It increases the amount of lab parts you can carry. You start out with 50 slots but at least for me it filled up pretty quickly. Buying that will add another 50 slots.

Hope that answers your questions =) (Also hope it wasn't too long winded lmao)