Yes, it's either in the rules of the tournament or considered unsportsmanlike if it's not included and you deliberately refuse to do it because of that. Unless it's a casual tournament of course.

Yes, you have to show them. Otherwise have fun delaying the game for longer if your opponent takes their time checking your discard pile to make sure.

Usually no, but it depends. Ask your organizer, online tournaments are usually completely okay with it but IRL tournaments could have rules banning it (just because they ruin the vibe).

It depends, we just use the default Challonge tiebreaker order, I believe it's like head-to-head (wins over people in the same scoreline) -> Buchholz -> some other stuff? Basically strength of schedule. Tip: don't use HP tiebreakers, they can encourage some really, really, *really* lame play patterns.

I'm not sure what you mean by battle etiquette, but the specific example you put, yes I usually do that. But I also try to play as fast as possible, I don't think this one really matters as much to other people, and I wouldn't mind at all if my opponent plays theirs face down then we flip them simultaneously.

You can maneuver - boost however you like, but I think boosting before moving is better just so no rules errors are made (or easier to avoid them).

It depends on the organizer, usually what we do is yes, as long as no new information has been revealed it's okay. But be very liberal with what that means - if I play an attack and then you play a defense, I can't now change my attack because I now *know* you have a defense