I had a house in Cape May New Jersey it was built during the Civil War.

Don't know if there is a private alternative for enforcement (like a citizens' arrest, although this is civil).

Would you imagine a licensee with the intent to purchase a property at a “wholesale” price could double close and re-sell without much litigation threat?

Sounds like a liability so I would get the survey and if it is on your property remove it and avoid future litigations. 

This puts law enforcement behind them and I'd imagine that it would sound really good to the jury in their civil suit if they could say that there was an enforcement action by the feds. 

If that checks out I would reach out to a local civil engineer for guidance on feasibility and assistance on the subdivision.

If the squatter can produce a piece of mail or an electric bill that has their name and address on it, then "They live there" as far as the police are concerned and it is a civil matter.  

When it comes to conversion litigation, having a knowledgeable legal team on your side is essential.

A civil engineer can help you out immensely.

I enquired about a litigation attorney and its $1K to even speak to someone.