The fact that he broke the agreement and is asking for a change isn't unusual. Just because someone asks for a change doesn't mean they will get it. Your expenses have gone up with regards to the kids and I would be sure to have your attorney document those costs - school, braces, car, etc.
Has there been any significant change in circumstances for you or your ex? Could he point to a loss of income as justification for asking for the downward modification? If so, he could get it, and you would have to adjust expenses where necessary.
The transportation issue? Assuming that one of you moved a distance away, that person is normally deemed responsible for ALL transportation costs. Search the forums for 'move away' to see more info.
As for increasing visitation, courts give the kids minimal say, if any, whether or not they have to spend time with the other parent. Activities and teenager social lives are normally deemed low priority to the parent-child relationship. You can ask for a phased-in approach if he's not been active in their lives, and ask that he allows/transports kids to their activities if they occur on his time. Note I said "ask" - he doesn't have to agree or comply.
In prep for the mediation, I would make a list of what you're willing to compromise on and whats off the table. Make sure that your attorney understands your position very clearly and keep in mind that if either of you push it to court neither of you will likely be happy with the outcome.
Marriage doesn't come with a money-back guarantee.