best settings to save data iPhone 4s (newbie user)

I'm an iPhone newbie (ex android user) and I'm having some trouble figuring out the best balance between keeping a low 3G data usage and being able to use the phone as what it is: a smartphone (not just for calls and texts).

I just got an iPhone 4s with AT&T and the 200MB per month data plan. The main reason to get this plan (or at least that's what I thought) is that since I work from home, I spend most of the day there (thus with a WiFi network always availble), so I shouldn't need to use that much 3G data in general.

Well, here's when things start to be not-so-pretty. The first thing I've realized, and investigated a lot online, is that the iPhone, while in stand-by mode, switches back to 3G instead of staying in WiFI mode. However, this does not happen if the iPhone is charging. That means that, during the day, even if I'm at home, it keeps using data from my plan when I receive emails, text messages from iMessage or Whatsapp, facebook notifications, etc.. Because obviously, albeit being a cool phone, sometimes it needs to rest and stay in stand-by mode.

If there's anyone who knows how to avoid this automatic switch to 3G data, please let me know, but I've done some research and the only solution seems to jailbreak it and install one app that let's you do that, but I would like to avoid that, at least for now.

Well, continuing with my concern about how to balance the data usage and settings in the iPhone:

After realizing all that, next thing I did was to install the Data Usage Pro app from the AppStore. It doesn't save data, but seems pretty good at tracking the amount of data you send/receive both in 3G and Wifi. At this point is when I start seeing things that I really don't like.

First, trying to reduce my data usage, I changed all my email to manually fetch. Also, turned off push notifications for most innecessary apps. But, after doing all this, I can still see little amounts of data being received and sent during the day (at home or outside). I wouldn't say now this is really bad, after doing this switching off almost everything it seems to have helped, but...

Second, huge data transfers at night. Yeah, after having almost everything switched off, also having said NO when you set up your phone to help Apple to bla bla bla sending usage data, the phone with the last software update, etc... I realize this moring that during the night my phone received 280 MB!!!!! of data and sent 6 MB. How is that even possible? Luckily for me, I always leave my phone charging at night, so I guess that's why it was on WiFi mode and all that data was received through WiFi. Had it not been charging, it would have used my 3G data, meaning that in one night I would have gone past my data cap (so $15 more bucks for AT&T).

Is this normal? Is there any way to avoid these things? I would like to know what kind of setups other users with data usage concerns have and see if any would work for me. Right now, I'm a little diappointed, coming from an android phone where I never had to worry about any of this stuff and I received all kinds of emails, notofications, etc. I know that I could also upgrade my data plan, but with these rates of data usage, I don't think it would be enough...

Any ideas would be much appreciated! Thanks a lot in advance!