How to prove your being remote accessed ?

If some one is remotely accessing the indication for it is , take the cursor on top menu bar ( on the right extreme corner ) you will see two small squares overlapping each other .

To avoid it open system preferences > sharing > click on plus sign a window appears , select the name of the unknown person and click on minus sign to delete .

You can do the settings as per screen shot , uncheck all options , also change all your passwords .

You can read this article : macOS Sierra: Set up your Mac to be secure

Ask your attorney. Apple will not get involved.

The assistance you seek is legal, not technological, and the circumstances you describe will be difficult or impossible to litigate. How do you intend to prove that someone erased something from your Mac, when it can just as easily be posited you are the one who did it?

Since all I can offer is technological advice I recommend you disconnect that Mac immediately so as to preserve it as evidence. I recommend that you purchase a replacement Mac, set it up as a completely new one, with a completely new Apple ID, completely new email accounts, etcetera. That way, it will be as separate from the existing one as yours is from mine or anyone else's. Given physical access to a Mac, or administrator access to it previously authorized by an existing user, it is a trivial matter to create what is colloquially referred to as a "back door" allowing access to its contents equal to your own (with some esoteric caveats irrelevant to your particular concern).

Also, be sure to secure your Apple ID. Please read If you think your Apple ID has been compromised - Apple Support. Any Mac that has been secured by your Apple ID is only as secure as that Apple ID.

More: Security and your Apple ID - Apple Support

Disconnect from the internet, then clean house.

They can't and they won't, call the police, if what you claim is happening it's illegal.

Follow John's advice, preserve your Mac as it is (for evidence) and contact law enforcement. You don't have a technical problem that we can solve.

If you live in the U.S. call the F.B.I.