Qualifications = national exams - you enter “US high school diploma” as the general system but then you’d enter your AP (subject and score) with the date of the result as well as your current APs with “pending”.
I’m not sure wrt changing the statement after your recommender has sent their recommendation - you must complete your statement and send everything before Jan 15, and your recommender has till Jan 15 too. Your statement is different from the commonapp essay and is sent to all universities so it should detail how you’re qualified for the subject you chose. imho it’s not as hard to write as the commonapp essay.
Sciences Po doesn’t teach in English except for the 3 specific programs (“minors”) I listed and for the dual programs with the US/UK/Canada. You must apply to 2 minors or 1 minor and 1 dual program. Then they decide if they send you to Reims, Le Havre, or Menton. In your case, Reims would make the most sense. So it would be logical for you to apply to Reims (French/North American relations) as well as one of the above dual degree programs, UCL’s being the closest to your interest in the UK.