Just a note, ATPL is an European licence, ATP is the FAA one. If you read 61.153 there is no requirement to hold an American Passport, so that should answer that, as far as taking your written, it is a good idea to have it before an interview, but you also want to retain its knoledge, if too much time elapses you might forget most of the data, so it could e better to take it closer to interview time and use it as a study aid. If you can, take your ATP on the plane you are instructing, it will be by far the easier plane on witch you will have the most experience, unless you want to wait until you upgrade at a regional.
Good Luck
SK3