I'm just finishing up high school in the US, and I only applied in the UK but all my friends went through the US process so hopefully my answers can help!
1) I can't really help here as I didn't do A-levels, but I can't see that being a problem as the US doesn't have an equivalent system for GCSEs etc.
2) I took the ACT, but the studying is similar: get a practice book that gives you advice for different types of questions and make sure it's got 3+ practice tests in the back.
3) The SAT's actually changed this year (I think you'd be in the first year taking the new one) so it's a lot more similar to the ACT now, but the big difference is that the ACT requires more time management. It's really intensive and fast paced (the SAT isn't slow by any means, but it's more doable if you're a bit slower at answering). The ACT also has a "science" section. I'm putting it in quotes because it's not about actual science, it's really just logical/quantitative reasoning. Also subject tests are probably required, but check their websites (some colleges don't require them if you take the ACTs, but all that would be on their websites).
4) No idea what it's like for international students, but I would imagine it's similar to US ones where the Common App goes live in late August, you apply by October (don't hold me to that, it might be November) for EA/ED schools, and then January for regular decision schools. You hear back by the end of March and make a decision by May 1.
5) Definitely worth it! Ivies/Stanford especially have got good endowments so should have scholarships (again, check their websites for specifics).
6) I didn't apply to both, but my advice from witnessing friends applying in the US is don't put off the Common App. Stanford for example has five or six supplementals along with the core Common App essay, you don't want to put those off to the last minute.
Also, in terms of you being worried about your GCSE grades, US colleges love to see improvement, and it's not like your GCSEs were at all bad!
It's good you've got a lot of extra-curriculars and leadership positions in school, those all look really good on the Common App.