You are not alone, many people come asking here for Grad Nite opinions! This thread may be of some help from last year:
http://www.disboards.com/threads/what-to-expect-on-grad-nite.3403849/
I have traveled to DL multiple times over the last few years during Grad Nites. We hit 2 grad nites per trip every time. They were during the weekday as we left by Fri each time. My experiences as related to your questions:
-They are "hopper" tickets so they have access to both parks. With the party being in DCA most of the grads will spend the day in DL.
-We noticed them starting to arrive by lunch in DL
-DL was packed with grads by dinner
-I had heard the 10pm thing and was excited at the prospect of all the grads being cleared out of DL by late night, this did not happen.
Other tips
-Do NOT try seeing PTN's early show or the firework show on Grad Nite. (The late show of PTN was fine, we arrived after dinner via monorail for the early show and it was so crowded just getting to Main St and the kids were rude -pushing and shoving like- so we found a table at Plaza Inn and basically hunkered down for the next couple hours. They cleared out by the late show and that was a great time to watch PTN.)
-You could see WOC on a grad nite (the grads will see this during their party) but when we were there they only ran one WOC showing before closing DCA to the general public, normally during summer schedule they run 2. So no grads but still crowded as it was the only showing that night. If you want to see WOC that night make sure to acquire your FPs.
-Do NOT ride "big rides" at DL that night (Indy, Space)
-DO buy "hopper" tickets this way you can go to
Disneyland as early as you can in the morning and then hop over to DCA for evening
It is entirely possible to enjoy your time at DL during Grad Nites. I've done it multiple times and we keep going back so it must be ok

But we keep making mistakes, I hope you don't repeat them. So here are the dumbest things I've done on Grad Nites in recent years: a couple years ago we returned to ride Indy via Fastpass line about 10:30pm. Do not do this. Last year we returned to the parks around 10:00 pm to ride Haunted Mansion. If you don't mind LOTS of extra screaming this is ok. Last year we did the PTN debacle described above on the 2nd grad nite because we turned our brains off or something.
Reports vary widely on the behavior of kids. It's something to be aware of if it might disturb you or your traveling party. Personally the extent of what we usually run into is a lot of bad language when standing in lines at night in DL. The worst report I've seen here on the boards is ride photos not being for sale because kids flash the cameras or make rude gestures rendering Disney unable to sell the photos.