I disagree. "Come what may" is referring to the future, but "I have always been true to myself" refers to the past. "Come what may" is a very antiquated term, to change it so it'd all be in the past would be "Having come what would, I have always..." which sounds really strange.

It depends on the train, the country, and the train company. I have heard all of these in the US except "carriage," but that is used (I believe) in England. "Coach" can refer to other things and is a bit awkward so I would avoid that. But "car" and "compartment" are about the same, where "car" refers to the entire structure (including the wheels, undercarriage, etc.), and "compartment" is just the inside of the car.

PRACTICE

I am practicing my English cheerfully every day. Now I want to show you these sentences. I can say some things but some are not totally correct.

My special day

The sun's shining. What a lovely day!
I love waking up to birdsong (or, "to birds singing"). I  smell the fresh coffee filling the home that I live in and it's amazing. I can't wait for today.
I will teach painting to children who live in an orphanage.
I never look down on them because they are exactly like me.
It's a long walk to get there and everything I see is beautiful: the flowers, the people, the trees, and the clouds. The bakery smells good, too.
Last time, I met Piper. She's 9 years old and she is very smart. Piper was angry with me because I forgot the pink paint. Her sky could have been pink but she didn't have pink. Then she painted the sky blue like all the other kids and said it was my fault. She's keen on painting. (We don't use "keen" in the US, but they do in the UK)
The kids are really interested in everything. They are kind and full of joy.
Helping them makes me happy.

"what's mine is yours" is correct. "What's yours is mine" means the person is probably about to steal all your stuff, or something.

This is very much a slang expression. It wouldn't be used in any kind of formal setting. It does not mean "realistic" at all. It means "honest". I am a native living in Baltimore.

"I believe you" is when you believe that something someone is saying is true. "I believe in you" is when you think a person is capable of something- "I believe in your abilities". We use the same construction to say we "believe in" the existence of God. "I believe in God" means you believe God exists, whereas "I believe God" means you believe what God said.

I think this might be a joke referring to something else going on in the scene, because it´s just not funny. It´s definitely not wordplay. Maybe it´s supposed to be funny because the control freak has string, which was supposed to be a ridiculous thing to pack?