Wow I really admire your decision to self-study all the way, but I would guess you're in for lots of hard work haha.
Easiest to study for: no way to say which one is easiest. Some people just have better abilities in chem or bio. If your math was pretty good then maybe physics would be good for you. It's a personal choice, really! Go with the one you like the most, not which one appears to be easiest.
About online courses, I never used any so it's hard for me to say, but I would think that it depends on your budget. Stanford's EPGY is one of the most rigorous online courses, but they're also really expensive. If you're lucky, other parents will chime in here and give you advice!
Of course it's possible, but if you think your background knowledge is really bad, then I'd start from the basics first. Look for self-paced courses or go to the library and pick up a few basic textbooks. If you can't find American ones, go with the IGCSE syllabus (the UK one) and they have lots of practice questions. If you can master the IGCSE syllabus, APs will look fairly manageable! Science is about lots of practice and being analytical and stuff, so best of luck!