1 - php file names should be fine
2 - that's on a site-by-site basis - some accept it, some don't - makes it up to you to either use it not submit to them or to not use it.
3 - a 780 wide banner is fine. I think you'll find most place allow up to 950 wide (which equals 1020 on a monitor)
4 - you should have as many different templates as possible - I have 40 or so
5 - you can go with more - I submit one free site to 40-50 link lists - yuou just have to make the layout look decent
5 - not yet
1st off, look at your site in different browsers - it looks fine in IE, but it's a mess in Chrome
2nd, you're getting your feet wet, so there's no need to jump in the deep end - get about 20-30 sites submitted accepted before you worry about submit accounts.