I am a lawyer who used to do real estate litigation on behalf of plaintiffs full time. Now it is a secondary area for me. Below is not legal advice, just my quick reaction to scanning your posts.
Sounds to me like the first step is to see if your dispute is covered by your title insurance and if so invoke it. It might be worth hiring a lawyer just to do that. I have never done this before so no idea how complex it might be.
I would charge about $6500 for doing the following: (1) writing a demand letter (2) preparing the complaint (3) filing the complaint, paying the court filing fee of about $500 and process server fee of about $80 (4) negotiating a settlement if they want to deal at that point. If there are issues with threats and disruption to your property, maybe another $3000 for a temporary restraining order.
Most likely when your neighbor is served they will either see how much a lawyer would cost to defend the case, likely about $10,000 up front, and want to settle right away, or else if they are crazy then they may default or try to defend it themselves, in which case you will likely win (if you deserve to win) and be able to get a judgment or injunction that will be enforced by the San Diego Sheriff.
As far as the costs of taking your case to trial, with experts, jury fees, motion fees, depositions, defending summary judgment and demurrers, etc., likely around $25,000 at minimum.
