Hi 🙂
Firstly, I would like to say that your courage and commitment to your future and education are worth being proud of! Altough I am not writing the Edexel syllabus, I'll try to answer your questions as best I can, with the knowledge that I do have. I am also 18, also writing my IGCSEs after very little preparation time, and not from a country where this system of education has a lot of options. I am a private candidate, which means that I rely on books and in the cases of French and German, a tutor. So with that in mind, I'll try to answer your questions:
1) It isn't too late, the concept of too late being very relative. Your brain is able to form new connections between brain cells, and can change and learn throughout your life. It is therefore, not at all impossible to acquire the knowledge you want to: in your case, the IGCSEs and A levels that follow. You might face stigma surrounding this but let that not deter you, your path is your own to walk. It is also worth considering that a lot of students take gap years or change courses at university - if that is your goal? Persuing further studies does not require you to be a certain age - it requires you to be willing to learn.
2) Subject choices and A levels. I am a CIE candidate so I do not know much about the Edexel subjects, but a list of the 40 subjects can be found here:
http://qualifications.pearson.com/en/qualifications/edexcel-international-gcses-and-edexcel-certificates.html Regarding subject choices I would recommend the basics (English, Biology, Mathematics) and from there on for you to choose the things you have an interest in, after having read the curriculums / syllabus content. This is very important, as it allows you to get a good idea of the focus points in your subjects and the knowledge required - many students are unaware of their own syllabus content! The same applies to A levels, depending on whether you plan to persue a university degree. Some universities have subjects they prefer or require for certain courses, so it is a good idea to check their websites for more information. Certain subjects such as General Studies are not always counted as being 'proper' A levels, but otherwise doing subjects you are interested in and passionate about is always a good idea!
3) Tutors: Tutors are the people you work with in chosen subjects and it is important for them to be aware of your educational goals. You might find that not every tutor you meet works in a way that suits your learning style, and you can then decide whether it would be better to adjust or to try someone else.
4) Self-Studying: Isn't easy. Is actually, arguably, harder than studying with a school because you have to learn skills that are usually only expected of university students: such as personal time-management, curriculum planning, prioritising certain subjects etc. These are learned by other students but to a lesser degree than necessary to be succesful when studying by yourself. There are pros and cons, and both are worth considering.
5) Brushing Up on Former Lessons and Re-Learning the Process of Learning for a Formal Examination: For this I would mostly recommend reading. Reading as much as you helps with general knowledge, spelling, grammar etc. and keeps your mind active and healthy! IGCSEs are usually not in as much depth as the A levels, so I do think you can study them without too much revision of the Foundation Subjects - what is more important is that you are eager to learn and that you study for understanding. xx
Good luck! It is such a wonderful thing you are doing - education is worth it!