I fail to see where the OP has failed to try the things suggested.
He said he had turned off celle
Sorry to say your wrong the OP has tried things suggested but the options are very limited and most people are continuing with the same stuff "Delete facebook, Change the phone and pay more for data!" all of which the OP is not willing to do and personally i dont see why someone should be forced to remove an app, pay more or switch phones because of a data cap which is too small. He asked for help and advise but so far all he's had is constant nagging that he should do all the three above, this forum is ment to be a tech site yet none of the responses have been very technical or good imo. Also why should the OP be forced to sell something his wife likes or to remove an app that millions of people use or pay more for data?
Think of it this way you have the internet your service provider has a cap what are your options if your going over that cap?
1. Install an adblocker - Not possible without rooting the phone the OP's wife does not want to do this though i admit it does the phone no harm.
2. Dont use the internet as much - OP is already using the wifi points as much as they can, could they use them more? Possibly.
3. Get a smaller computer - Im saying this because peopel keep saying change the phone i dont really see how the screen size eats more data, though i suppose it could be image loading im not sure.
4. Pay extra for the data - Seriously would you stay with an internet provider thats going to charge you more? No, i personally change providers and thats why im with Virgin internet because they leave me and my excessive downloading alone.
Anyway i could go on and on and on like the energizer rabbit but, im out of this thread now. Its upto the OP to decide what him and his wife want to do but there options are still limited with the options given.
1. Change the phone - Wife done's not want this
2. Pay more for data - Why should they?
3. Remove an app they use (forced)
4. Change networks
5. ? i dunno i can not be bothered to read though again and im watching tv.
He's already refused to root the phone to install ad blocking capabilities, so that's out. Facebook is using that data no matter what. YOU CANNOT GET AWAY FROM IT. Unwilling to root the phone he has 3 (count them, 1 2 3) options;
1) Don't use facebook
2) Get rid of the phone
3) Get more data (or pay the overages)
He's refused to do any of them... Something's got to give.