I have not read all the posts, but perhaps I can add something helpful.
I have a similar problem with an abutting neighbor, except this guy said he would move his piles of construction material, bricks. old raised garden frames, just crap you throw "out back".
Well out back just so happens to be my land. Property is basically square, 24 acres, half woods and swamp/pond. One side faces small residential community with backyards abutting my land.
I did not have a survey done when I bought the land. Big mistake. Original survey was off 30 feet on one side and a nice wooden bridge over a six foot deep drainage ditch
all of a sudden was no longer on my property. I lost the bridge... Well part of that $3k surveying was confirming where the line was in the backyards of these houses, all of whom I bet thought their land went back into the woods. No, not one little bit. I actually own into some of their lawns they all have encroached. Plus of course they have thrown their brush and junk "over the line" since of course no one complained. One guy has an old boat in my woods...
I talked with the one fellow with his piles on my place two years ago and he promised to move them, just retired, would get to it.
I just know I'm going to walk back there and see nothing has taken place.
And it will cost me another thousand bucks likely to have that side of my property restaked with about three times more line stakes so there is zero question where the line is.
Big white plastic pipes, ugly but very visible. I can see none of this from my inner property, 8 acres cleared out of woods.
Then lots of pictures take...a letter to the abutting owner, 30 days. Then a filing for Trespass if need be.
And if someone puts me through that, I've got lots of rolls of barb wire in my barn I can string along that line, just like it used to be to keep the
prior owner's horses in and critters out. That will be one nasty fence, and I can't see it at all from my home.
Paybacks sometimes can be "educational".
I'd much, much rather shake the guys hand and thank him for doing what he said he would do.
I always want to believe in folks, and then you get played.
I think surveyors and fences solve all problems. Most annoying to have to shell out all that money just because
others are selfish idiots.
And if OP actually needs a drainage ditch there, well that
would be an excellent farmland oriented solution. Where I live it's all about moving the water off flat land, so
ditches are everywhere. No one is going to drive a tractor into a three foot deep ditch...