Yes, because every problem in the show is a result of her mistakes, and she has to pay for it. Until S3, that is.
He wasn't, he was interested in Jackie and viewed Star exactly like a little sister he had to protect. Until S3.
Tom believes he's entitled to things because he's a possessive (it's a joke because he's also a demon) and wants what he can't have. Until S3.
They weren't before S3, except for Ferguson and Alfonso, who Nefcy didn't want in the first place, and Sensei, who was Marco's end-point if he didn't start applying himself in life.
Not until S4.
It's not, but because she's cute and empowered it's okay.
Yes. Janna was originally Marco's little sister, turned into another student, but kept her mischievous, insulting personality. Pony Head was one of Nefcy's first creations and her favorite characters.
Watch S1, S2, and Battle for Mewni, read the comics, and read Between Friends. Leave it at that.