Respectfully, if they can't tell you why neither can we. But they are getting paid to do so.

Any lawsuit against Twitter would be considered frivolous and, yes, you could be required to pay their attorney's fees.

Use password protection. Speak to your sister. Not a legal issue.

It is discrimination, and is perfectly legal IMO.

You can hire a private investigator, but in all likelihood this is a medical issue and there is no one following you. Best of luck.

The cost of any such lawsuit would be thousands, and will take years.

Good news: You can quit and go to work somewhere else. Bad news: They can fire you. Bad news: You have no grounds for a lawsuit.

Most lawyers will tell you to ignore a civil demand letter.

This is a matter for your lawyer to handle.

Yes you can sue. No you cannot win.