I don't really think it's fair to be tested on formal and familiar when it wasn't in the lesson. I don't recall seeing it in the tips before the lessons.
Are the words "dych" and "dach" are interchangeable?
They have the same meaning, if that's what you mean; the difference is regional - some will use dych and some dach. dach is more common in the North, I believe.
But I think any given person will use only one or the other.
What is "wyt ti'n"? Is it a regional difference?
It's the singular/plural divide, kind of like tu/vous in French.
Wow, if I didn't read this comment, I would think exactly the other way.
What if I wanted to ask one stranger something? Would I use wyt ti'n because it's singular or dych chi'n because it's formal?
Always go formal with a stranger.
Syr, mae hwn yn becws.
Me too, so I should use wyt to ask one of my friends if they want e.g. a coffee..