Can my ex wife comeback and ask for back pay of the difference even though we verbally agreed on $1200 monthly and checks to prove it?

In Nevada she can ask for the difference between what you paid her and the amount you were supposed to pay her. The court will want to see a written agreement and Order.

If she accepted less you may be able to fight the arrearages but you should have put it in writing and then requested a modification from the court due to your change in employment.

Until the court order is changed, it stands. If you had an agreement to change the child support, it needed to be a written Stipulation meeting the requirements of the court, then filed, and signed by the Judge. Once that happens, the support will be changed, but not until then. You will want to file to modify the CS as soon as possible. Before you do that, you may want to have a Stipulation to modify the CS written up properly and commencing on the date you agreed, then see if she will sign it.

I need more details and you need proof of the agreement. . If you are in Michigan call with details and for an appointment.

Unfortunately if you have an actual court order (or even an settlement agreement that was then incorporated into a court order - i.e. signed off by the judge), then those are the terms that govern. If the parties agreed to something else outside of that, that's only good for as long as neither party then tries to enforce the underlying order later, especially if the subsequent out of court agreement was only verbal. The evidence of the diligent payments to her all along are certainly better than cash, but that only proves you paid less than what the original order was for, not that you were allowed to under pay or that she agreed. But if you have any emails, text, and other helpful evidence, that would certainly be helpful if you find yourself back in court because she now wants to enforce the original order.

Yes, she can claim that you owe arrears under the court order and come back for it, and will likely win along with interest on the arrears and penalties. The court order controls, not a verbal agreement. If she will still agree to accept $1200 based on your lower income, get that put into writing and reduced to a court order, to include the past years.