DIY surveyors transit

Rusty,
I think you've hit on the solution, mark the boundary conservatively so that where there is uncertainty you make sure your property is protected. Its not like what you're doing is a legal survey. That puts the onus on the neighbor to survey the land if they dispute the boundaries. After all its their actions not yours that have resulted in your losses (time, money, property damage).

Edit: It if was me I'd only hire a surveyor if the neighbor agreed to pay or share the cost.

I can't let this go ... I thought of one more way to mark the line. Get a bright 12v spotlight (maybe wheelbarrow a car battery out there if need be). Shine it straight up into the air from one property corner - kinda like when a car dealership has a searchlight set up for a "grand opening". Guess it'd have to be pretty dark, but not too dark to somehow mark a point that's along the line (I just need one, near the "midway tree" that I've mentioned before and I'm done); probably just have girlfriend, with a flashlight, to define the line. Also, if the air is too clear, I guess the beam won't be visible; need a foggy evening ...