The officer was wrong.
The officer might not have been able to make an arrest because he did not see the assaults depending on state law. However, he had EVERY reason to believe the OP and he SHOULD have trespassed the other people. That does not mean the officer had to arrest but just tell the people to leave and not come back. This depends on state law and department policy. If the officer did make an arrest, let the judge and/or jury figure out who owns the property. The officer handled this situation poorly. He had every reason to believe the OP and none to believe the mob. The officer has a family with children surrounded by a mob. Duh. That situation needs to be descalated.
Depending on state law the OP might be able to go to the magistrate and swear out warrants on the people who assaulted his wife. It appears the OP has witnesses but will they testify?
The cameras are a must in this situation. At a minimum, pulling up the the fences and taking down the signs is a destruction of property and/or theft. Cameras that cover the property markers would be a very good idea. I suspect they will go after them next which is another violation. Given the seriousness of this situation I would be talking to the officers supervisor or go to the Sheriff. Not quite sure how I would handle this one. Start low and work your way up or just go to the top.
Good Luck,
Dan