What exactly do you need assistance with in terms of proofreading/helping? Syntax, grammar, expression, ideas? I am happy to look over your essay for you.
Depends, is there a reason you're doing the past papers? i.e. studying for a test or studying beforehand in general? If it's for the first bit, then focus on the stuff you know. If it's the latter, just try as much as you can. It's good to expose yourself to a variety of different questions even...
My school uses it too! Atomi I think is good for basic level revision and repetitive practice mostly. The videos aren't too bad for a basic foundation and its good that they have a lot of topics on it but it can be quite simple at times