When Does a Modification of Child Suport Take Effect

My question involves a child custody case from the State of: TN
Oct 31 2012 I received papers stating that my ex no longer wanted to exercise his visitation of every Thursday, Holidays, 1 week at Christmas, every other fall/spring break and/or 3 weeks in the summer. He only wanted visitation every other weekend for our 2 kids. One of the kids turns 18 this year in Aug. So, I got an attorney and we started submitting paperwork to get it modified and for him to pay back doctor bills that he refused (and I was told that i needed a significant amount of money before taking him to court or it wouldn't be worth it - or we would be in court all the time for copays etc..) So, I submitted those along with my documentation of salary. My ex and his lawyer didn't get back to us until the last week of December with his salary information for us to do a modification. We did the modification and his lawyer said she would talk it over with him and get back to us. Jan came and went and in Feb we inquired as to him signing the modification papers and they hadn't determined how he was going to pay for the old doctor bills yet. This has gone on where he says that he is thinking or figuring out how to pay what he owes in back bills and yet he is not signing the moficiation that HE REQUESTED, which of course increases his child support. Now it's May and it's still not settled he is not signing off on anything and my lawyer filed the petition in the court in April.
My questions are these:
1) Am I able to get back child support starting in November when he requested the modification in the parenting plan or is when the petition is filed?
2) If it's from when the petition is filed is there something I can file because he requested the modifcation not me and he is trying to get out of paying the increase and he is trying to wait until our oldest is 18 in order to get out of the extra payment.
3) How long does he have to review the doctor bills since they are 3 years old without penalty?
4) At what age can kids they say they dont' want to go and its strictly child support and no visitation?
5) He hasn't covered insurance on them like in our papers either and that started in Jan.

I would like to know what I can do through the court to get him to sign/pay and continue to pay for what he should be paying.

I am at a loss at how long this has dragged out. I know that my case isn't high profile or anything but there is no reason that a modification that was requested by the dad should drag this long because he won't sign it due to the child support amount?
Thanks
Frustrated77

Modification of child support can normally be retroactive to the date the motion for modification is filed.

If you submit three-year-old medical bills to the other parent or the court for the first time, the court may find that you've waited too long. If you seek enforcement of a bill for the first time three years after submitting it to the other parent, the same thing is possible. You should discuss the facts with a lawyer.

The kids can refuse visitation when they turn 18.

If your ex- has insurance that can cover the children consistent with the court order, and is not doing so, you can attempt to enforce that provision of the judgment.