Parcel Encroachment On Setbacks And Septic After Survey

Hey all, I am in Location: Kalkaska, MI. We bought our house in 2020 and, while going through due diligence were told that the our parcels(1.5) ended at the tree line behind our house. Unfortunately, we took the agent and maps at their word, and did not have a formal survey at that time.

Fast forward two years and the neighbor behind us informs me that he thinks the property line is right up to our back door and he is going to have a survey done to check. He ends up being more right than either of us expected as the corner of his parcel is about 4-5ft from my back door. His parcel also has a large portion of my septic drain field on it. Both of these things are noncompliant with local code according to the zoning ordinance for the county. Depending on the classification it's anywhere from 10-30ft for setbacks and all of a septic system has to be on the parcel it serves.

He has proven to be uncooperative in trying to remedy the issue. I've offered to swap some of the land that runs along the southside of his property to adjust the lines and get things into compliance, he refused.

I offered a fair price based on local averages(probably more than i should have because I was still treating the land as "market value" at that time which I have learned is not the case) he did not agree to that price.

He has since countered with an outrageous per square foot figure that is outside anything that would be considered normal, average, or fair for the area. Especially considering this strip of land would be characterized as "nominal" according the the reading I've done up to this point which greatly lowers it's value in transactions like this.

I have respectfully(not kidding here, he's a drunken asshole who calls me names and all kinds of other stuff but I've been very professional and never engaged in negativity with him at all) informed him that I wouldn't be paying him what he wants for the land.

He is now threatening to "build a fence right along the property line" come springtime which would go right through the drain field and be within a few feet of my house.

I have since dug up how this situation was actually allowed to occur in the first place:

-the parcel my house(built this time) is on and adjacent parcel were platted in the 60's as-is

-the septic was installed in 2009

-up until 2016 both parcels were under the same owner which defined them as 1 lot for zoning purposes (section 20b in the ordinance) which is why the septic and property line were non issues

-in 2016, the neighbor bought one of the parcels from the owner thus reclassifying both as independent lots under section 20a in the ordinance. The ball was dropped here. I can't find any paperwork whatsoever that anyone did anything to make sure that these two parcels were compliant at this point. I've talked to every agency that would potentially have records, none do. Neither are now "buildable lots" according to my reading of the ordinance.

I have tried, unsuccessfully, to have the zoning admin for the county tell me 1. if the parcels are compliant(I didn't want to assume anything or attempt to do her job for her) 2. what my options are for bringing them into compliance if they are not. She has declined to do either or even engage with me at all to help.

My questions are as follows:

1. My reading of MI law tells me that because of the representation of the property line as the tree line--a physical demarcation where grass was mowed along and obviously being used as the property line between the two parcels--by the agent and the maps we were shown. I may have an argument that not having the survey done at the time of purchase would be considered reasonable reliance by a court. Is this true?
2. What actual legal options do I have here? I've read the county can "suggest" a boundary line adjustment that would bring both parcels into compliance but they have no authority to force him to agree to it. They could deny permits for anything he would want to do with the land but that would require the zoning admin to involve herself which she has patently refused to do.
3. Because both the house and septic system existed, compliant with code, before the "triggering event" sale in 2016, do both get grandfathered treatment in terms of remedial action regardless of what happens moving forward? Essentially, I'm asking if there is any eventuality that would require ME to move the septic or something like that?

Appreciate anyone who takes the time to read through all of that!