Don't think the "farmer across the road" is too sharp.

It has been 7 years since the agreement was made and he allowed the drain tile installation.  In those 7 years not one peep about water problems etc.  Now he has complaints..??? 

As to the harassing letters,, whoever has legal authority to make decisions about the ground in question is who should be receiving the letters.  IF your cousin does have that legal authority, he was correct sending the letter advising "donkey across the road" to send correspondence to him and only him.  Given that,, "donkey across the road" is indeed engaging in harassment of your Aunt.

From your post,, it looks like "donkey across the road" has some sort of heartburn with you more than either your cousin or Aunt. ( somehow,, you ever whizz in his cheerios at some point in the past?)

If it were me I would put on my "nice hat" go talk to the guy and try to figure out what his real issues are, and try to mend the fences.  Ask him what the problem(s) is he has with the drain, etc..  You can catch more flies with honey than with vinegar..

After talking with him I would also sit down document, Date-time- and exactly what was said by both him and you and hang onto that paper!