I've been doing research on the internet, but I seem to be going in circles and I can't find the specific answers I am looking for. Hopefully someone on here can help me Here is the situation:
This is in Florida. My brother has 2 children with his ex wife. She is the CP and my brother is the NCP. They own a house together which has been on the market for almost a year now (since the divorce was finalized). She is living in the home with the 2 children while it is on the market and my brother is paying the mortgage payment every month ($2300/mo). The divorce papers say they will split the profit once the house sells. The ex was not working at the time of the divorce, so when the child support was figured my brother was told to pay $1200/mo for the 2 children based on the last 3 years of his income (he owns his own business). My brother and his ex made a verbal agreement that while the house was still on the market, and she was living there free of charge, he would pay the entire mortgage each month and start paying her the child support once the house sells and she moves out. This was actually her idea. Also, he was living in the house (sleeping on the couch) with his ex and the 2 children for the first 6 months after the divorce was final because they both figured the house would sell rather quickly, which it did not. Now, the ex demands money every month above and beyond the mortgage payment being paid. (my brother buys the children new clothes and toys when they need them and when the mother doesn't have a lot of food in her house, he goes to the grocery store and fills up her kitchen.)When she is reminded of the agreement they made she threatens to go to the courts and have him put in jail for non payment of the child support. My brother does not make anywhere near as much money as he used to and cannot afford to pay the child support and the mortgage on this house. The ex is now working a full time job as well as some part time work from home job. Here are my questions:

1 - I read somewhere that since he was living in the house for about 6 months after the divorce was final, he is not responsible for any back payment of child support for those specific months since he was paying all the bills and the ex was still not working at that time. Is that correct?
2 - The ex now has a full time as well as a part time job, so my brother can file for a change in the child support amount he has to pay each month, right? It has not been 3 years since the initial filing, but I believe you can do this if there is a "significant" change in the circumstances of one of the parties. I would think this would qualify Do you have to get a lawyer to do this or can you just file the appropriate paperwork?
3 - Since my brother owns his own company, is it correct that they will base his current income, used in the calculation, on the average of the last 3 years of his income? Or will they accept some sort of proof of what he pays himself for the current year only?

He has no desire to take anything away from his children and will always provide for them anything and everything they need. But his ex is very shady and has even told him he is her personal ATM and that she can take whatever she wants from him because she got custody of the children. I just want to make sure he pays her only what the law says he has to, and he will be sure the children have everything they need. I know that has nothing to do with my questions, but I know how a lot of NCP's are viewed in a negative light and I'd hate for anyone to think my brother is like that and therefore not help us out with these questions he has. Thank you for any help you can offer us