1) The cosplay square closes earlier tomorrow, yes. Generally, it's always open as long as the market is open. (Please make sure you ask cosplayers before taking photos, and only photos outside is allowed.) Read this:
http://www.comiket.co.jp/info-a/TAFO/C83TAFO/C83eng.html (CTRL+F for "Anyone who wishes").
2) What exactly do you want to buy? If you're looking for specific limited items from industrial booths (like artbooks fro Type-Moon, SHAFT, etc), then there are people lining up even from the night.. It is extremely hard to get those items if you show up too late. Even if you show up early, they might sell out before it's your turn. If you hear the start of the line clapping, that means they've sold out. If you don't mind hours long waits, then 8 AM should be okay. Just make sure that the stuff you want is still there on Day 3. A lot of booths sell out the first day. Trains start around 5 AM, Comiket opens at 10 AM. It's up to you how you wanna do it. I don't wanna tell you to show up at 6 AM and then you waste 5 hours in line for an item that sells out right before it's your turn. So please be aware..
My biggest advice would be to know exactly what you want to buy, and where it is located. All the maps are Japanese, so you need to write down the Kanji for easy/west etc, based on where the booth you wanna visit is. If the doujin you want to buy is not so popular, then regardless, I suggest you check tonight on their Twitter (if you can find it) if they sold out today or what. Again.. it's difficult to say because you never know when they will go empty. But a lot of doujin circles go on Day 3, so there is still hope. If you want from popular ones, maybe 11 AM would be good. But don't take my word for it - it all depends on the situation.
If you're not looking for something specific and just wanna browse/buy on impulse, then anytime of the day should be alright. But, it's actually advised you show up later if that's what you wanna do. Maybe around noon.