To quote multiple posts, press "Multiquote" at the bottom right instead of "quote," and press it for each post you want to quote in the order that you want to quote them. As you press each post, there is a counter that should go up, indicating the total number of quoted posts. When you're done, there's a button to commit to quoting, and each post will appear as its own quote in the order you clicked them.
There isn't a shortcut way for editing quoted posts that I know of. You'll just have to manually capture the text that you want to delete. If the quote I want is in the middle, I usually start just before it, and capture all the text to the top of the quote box. Then I'll delete. Then I'll start just after what I want to quote, capture all the text to the bottom of the quote box, and then delete. Be careful not to capture anything outside the quote box (it can take a little practice) or you'll accidentally delete the quote box, itself.