I can answer some of your questions:
You should send an email to the U.S. Embassy in Lagos at lagosIV@state.gov to ask when your interview date is. They will reply you within a week with your interview date and packet 3 attached. They will also include further instructions regarding medical exams and how to pay the $240 fee (I believe it can be paid in dollars or naira). I have read that more recent letters of intent you and your fiance provide should be sufficient to prove engagement (in addition to photos, chatlogs, emails, etc).
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