Two questions:
Is this a subdivision?
and
Have you had a survey done?
Two questions:
Is this a subdivision?
and
Have you had a survey done?
I'm sorry to be the bearer of bad news, but you are going to have to hire an attorney. You may be able to have your attorney shut them down early when he asks for a deposition from the back neighbor, by clearly demonstrating that they don't have a chance in heck of winning, but probably not. It boils down to deciding how much you want that property.
What is the status of the "critical wetland area" ?
Corps of Engineers, DNR, whatever? What effect does this have on your situation?
You make this out as a spat between surveyors which you seem to have some insight in deciding. Extremely accurate measuring does not equate to surveying.
You are free to engage a surveyor of your own and that would likely be the least expensive method of determining your actual situation in this matter.