As for the top or bottom of wing first, I don't know how you would accomplish putting the metal pieces at the lift strut brackets if you put the top on first.
You can use waxed string to secure stuff if you were a scout or Navy guy who can tie knots that won't come loose. I was neither.
Your access cover is about like mine. I used a edge bender to put a slight angle around the entire piece and it comes out flush against the fabric. Looks nice. Not necessary.