"I got x, should I keep the account"

Really depends on how much you're willing to mald, the only thing that will actually make a difference long term would be a magical 2lb or higher fine motion or super creek, but that is an extremely low probability event. Practically speaking you're looking for some speed cards to tide you over until kitasan speed, a finemo/creek + any SR or better speed card is perfectly playable

Should I use the 3* ticket on X

No. Wait a week, you'll get a free random 3* character from the new player missions. You really do not want to whiff and get a duplicate character.

How does X game mechanic work

Don't worry too much about it at this stage, just play for fun for now. The first major pvp event won't be for a few months so you have plenty of time to mess around and have fun. If you must know, there are new player guides available in the pinned messages

Who do I train?

Train who you like. Or if that isn't good enough for you then just try and fill up your team trials team and increase your team rank (this is a huge part of your new player jewel income and also a good yardstick for your progression). Don't worry too much about training better parents right now, it doesn't matter at the start all too much anyways

Should I keep pulling (after initial 40 pulls for rerolling)

Probably not. It's not a bad idea to do a small amount of rolls in the uma (character) banner (I'd put in around 20 personally, but the less the better). The idea is you get enough of the 1* and 2* characters to fill out your team trials team and nothing more. There will be a meta-defining (overpowered) support card coming in roughly a month, and any pulls you don't put towards it will directly reduce the odds of you getting a MLB copy of the card (important).
Or you could also just be a degenerate gambler and keep pulling, I won't stop you . But it is quite suboptimal