"It's hard to live a day without you, but I'll bear it and remember each day, one by one, since I started loving you."

As far as I know, 행보 means "move" as in a decision. For example "addressing the situation politely is a good move" or "raising the advertising budget was a good move for the company."

They can both be used to ask where someone was before arriving at the current location. You're right though. "Where did you come from" can be used to express the feeling of being surprised by someones sudden appearance. It's all about how you say that phrase and the situation it's being said in. Generally, they mean the same thing and can both be used.

They're can both be used in this example to express sarcasm.

"Whom" is just a different version of the word "who." They can be used interchangeably. Although, "whom" sounds more sophisticated and is generally used when speaking formally. "Who did you watch the movie with?" "Whom is it you are looking for?"

The term "stand-in" usually refers to someone as a substitute for something.

The word "whom" doesn't make sense at the end of the sentence. It would be "Whom do you talk about?" Other than that, yes the sentences are similar. "Whom" is just a variant of the word "who."

It as simple as wishing for someone to have a good day. The speaker is wishing for the listener to be happy on whatever day of the week is specified.