We are in the US and DD1 is applying for colleges this fall, and we have done quite a few college tours recently. I haven't been through the US college system, so it's new to me.
OP I think you'll find your visits answer a lot of your questions. I found them really useful. Have you scheduled tours at the colleges you want to see? I found them very informative. Where are you looking?
I can't answer many questions about how to find info if you're an international student, although I can tell you that many of the colleges we visited treat international and home students the same when it comes to availability of scholarships and funding.
It does seem that to some extent you don't know how much college will really cost until you get your offer, and even then you can negotiate the cost down if you've got other offers - that's true for students applying from the US. Very few of my friends have paid full whack, especially at those $75k a year colleges.
Some of your questions...
3) How did other parents/students go about getting sponsorships/scholarships- and when did they start applying?
The main help you are likely to get will be in the form of aid from the college itself, and that will be decided as part of your college application, based on your income, how badly they want your child, etc. We apply for most financial aid here using the FAFSA form. I would assume you can't fill in a FAFSA without a social security number, so when you visit the colleges ask them specifically how you will submit your financial info to them and whether you should use the FAFSA.
Separate to that, there are various websites you can find that list lots of scholarships that your student can apply for. It's their job not yours BTW. Your child can start as soon as they like - DD is starting now (she's going into her last year of school in Sept). There are lots of little bits and pieces scholarships here and there.
4) did you have Skype interviews or just offers?
The impression I get is that all applications are read, often twice, and that interviews are much less of a big deal than in the UK.
6) did you sit the SAT or ACT, which one did you choose and why?
7) when did you sat it, how many times and did you get a tutor? Do you need a tutor? DD looked at the maths one and said it was easy hence the comment about them being easy. Which specialist papers did you do?
In our state the SAT is more common, and it's what DD's school offered. She took the PSAT then the SAT. She got a high enough score on the SAT that she's not going to take it a second time to increase her score. She didn't have any tutoring but she's ridiculously good at math and english so it wasn't really necessary. I'll probably (all right, definitely) get tutoring for my other kids. She's going to take some subject SATs in her senior year - maths and some sciences - because she wants to do maths at college.
You can increase your score by studying hard - one of her classmates studied an hour a day all summer and increased her score significantly.
8) how much assistance did your school give and did they understand the reference and school transcript requirements- if not how did you get it done?
US universities are used to seeing GCSE and A level results in transcripts. There's a lot of info online about what should be included in a college letter of recommendation, which you could give to the teacher/employer writing them for your child.
Look through the websites for each college your child is thinking of applying to - they'll talk about what they're expecting in terms of essays or other materials.
US colleges are not just looking for clever/high achieving students. They're looking for students who've done well considering where they come from, who've shown grit, who've shown passion in pursuit of one particular thing, but who've also shown a range of interests. Bear in mind US students have to log dozens of volunteer hours to graduate high school, so they'll want to see your child having done a fair bit of volunteering.