Here is my concern. If you don't get this resolved, he IS going to get extended visitation. I think you need to push the mediation harder.
He can't just take the kids for 4 weeks. But, if he goes into mediation in September, then it will work against you to not have given him any.
I know the abuse issues alone warrant a more permanent solution, but no judge is going to say that they agreed to extended visitation one year with an intention to not have any the following - it doesn't follow due process.
I think you need to answer on the issue of mediation only and tell him that he needs to schedule it immediately to resolve the summer vacation.
I guess what I am thinking is that the documented abuse is still an allegation to the court. I am not sure of the status on all of the stuff you have been through. But if you don't mediate, it all becomes weakened as evidence.