A few questions on using iCloud to locate a lost iPad...

Question

I've read the posts on a lost iPad, and why it might show up as "offline" when trying to locate it, but it seems like iCloud should be able to identify it by the serial # even after a thief has reset it.  Could a thief also reset the serial #? 
Another question, I know it was turned on, and I know it had plenty of juice in the battery.  I had the 4-digit passcode feature working.  Does that mean that if someone doesn't actually punch in the correct code, and then log on to a wi-fi server, iCloud will never see it?
You know how when you lose something you backtrack in your mind to where you last saw it?  Well, I've done that, and there are only 2 possibilities. 1: It was stolen from a locked vehicle in a parking lot with lots of surveillance cameras, and the thief left behind a couple other things, way more valuable, and less easy to trace, and the vehicle was left undamaged, and still locked, or 2: I left it on the roof of the vehicle, and it fell off a few miles down the road. 
I think possibility "2" is the most likely scenario, so that leads to 2 more possibilities, 1: It got destroyed, or 2: It's lying next the road, working ( i have it one of those "ballistic" cases which I've inadvertently tested a few times, and they really do work quite well), but it's not logged onto a local wi-fi a few miles from my house.  Well, obviously, if it's destroyed, it's definitely "OFFline", but if the second possibility happened, is there any way that iCloud could find it?
Sorry this is turning into a short story, but in thinking about this, it's led to some other questions...
If it fell behind my dresser, and is in "dormant" mode (i.e. No one has punched in the passcode), can iCloud find it?  If the answer is "no", want if I got rid of the passcode screen, would iCloud be able to find it?
If a thief got it, and methodically went through all the code combinations (there are only 1,000. How long could that take?), and got to my e-mail, could he download all of my IMAP e-mail?  Is there someway to keep people from accessing an IMAP e-mail account once they're "in"?
I guess what I'm saying is, if I'm giving Apple permission to follow my iPad, and presumably ME, all over the planet, but iCloud can't find my iPad unless I, or some thief, enters the passcode, and logs on to a wi-fi connection, then what's the point of having "Find my iPad"?

Answer 1

Also...
Are you saying that in the "behind the dresser" scenario, it wouldn't be connected to my home wi-fi until I punched in the code?  In that case, I would have already found it, so no need to use "Find My iPad".  How about this?  I get rid of the code screen, so all I have to do is punch the button to get it out of the "dormant" mode, and go to my main screen.  When it's sitting there "dormant", (with a black screen) is it still connected to my wi-fi, at least to the extent that iCloud could find it?  If my new iPad is 4G, and I set up a data plan, would that make it easier for iCloud to find it, or does it only work with wi-fi?
Is there anywhere in the iPad e-mail app that would make it so I'd have to put in a password each time I wanted to access my accounts?  I'd try to figure it out myself, but obviously I don't have mine with me right now. 
As I understand it, now that I've gone on my PC, and used the "Find My iPhone" app, my iPad will be locked as soon as someone connects it to wi-fi, and it will stay locked until I log back onto iCloud, and unlock it.  Is that the case?  Also, will I get an e-mail if it gets back online, or do I need to just keep checking?
Thank you for the reply, and also for correcting my math.  It makes me breath a little easier.  It was only gone for about 12 hours before I locked it.  However, something just occurred to me.  I heard a true story about a hotel where someone kept breaking into the little safes in each room.  It turns out that a bellhop was putting some substance on all the keypads, so when a person entered their personal code, it would show him the 4 keys that had been touched.  With that information, he knew the the four digits. He just had to get the order right.  I'm not going to embarrass myself trying to do math again, but it greatly reduced the amount of time it took him to get in the safe.
If you look at your iPad from an angle (I'd do it with my own, but...), I'd be willing to bet you that there are some perceptible "dots" on the screen, from the oils in your fingertips, and since the keypad is above the keyboard level on the screen, you might be leaving hints as to what your code is.  I've thought of a couple ways to remedy this (one's really cool), so if anybody from Apple is reading this, get back to me.
Thanks again for the help.