'US' must be captialized, but for the 'the' you should only capitalize it if it is at the start of a sentence (Eg: The US is a big country).
Yes, US mail should be capitalized as it is an abbreviation for United States mail.
The word citizen does not need to be capitalized.
Yes, "Southeastern" in "Southeastern US" should be capitalized as it is part of a proper noun.
Yes. US Government is capitalized.
Yes, when referring to the title of the President of the United States, it should always be capitalized.
It depends on the context. If they preced the name of a continent they do.
No, but "the" typically is, so the US is written The United States of America.
Yes. It is a proper noun.
No, "sheriff" should not be capitalized unless it precedes the name of the specific sheriff ("Sheriff Johnson served us with a subpoena").
US with both letters capitalized means "United States". The word us with neither letter capitalized means me and at least one other person.I live in the US.I went to the store to get us some groceries.
No it shouldn't be capitalized.
To make a short answer short: No.