property line dispute/legal issue

I would not promote hitting a lady anyhow. I would however, devote my life, to the little things, like TP the place! Glue mailbox lid open, little things...

Ahhh, the little things in life bring us such joy!

I read the whole thing, I've not had experience in this particular area BUT I have lived for 64 years, in California.

Best advice you've gotten so far IMO
 
Is talk to a lawyer, if you've been contacted by a lawyer it really is in your best interest to go to one yourself for advice.  From experience, that would be the first thing I'd do.

As for a survey, there is nothing more valuable than accurate information, it would be great to have it but I imagine it would be costly, see what your lawyer says.

Not doing what the letter says may be OK, but not seeing a lawyer wouldn't be prudent IMO.

Good Luck, good neighbors are worth their weight in gold, bad ones unfortunately can really be damaging and costly.