Yes, the USCIS can and no doubt will check your address.

There is no average wait time, it varies extremely widely.

If you are naturalizing inside of the U.S., you must live in the jurisdiction three months before you file. But if you are naturalizing from...

There is no rule for getting paid for prior work, from outside of the U.S., while a person is visiting the U.S.

EAD Card is not a nonimmigrant status. There is no step you need to do to transition.

If he or she has a green card, you could just notify them that the person already immigrated.

There is no requirement to use an attorney to file the form.

You should contact a Costa Rica immigration lawyer if you have questions about who can be admitted to that country.

Check the USCIS website for who must collect I-9 forms. Generally, you must have a minimum number of employees first. So it depends on the size...

The correct answer is that it is anybody's guess.