You should consult with an experienced tenant’s rights attorney. Use the Avvo search tool to find an attorney. https://www.avvo.com/find-a-lawyer

You have, potentially, several who could be responsible. Consult with an experienced personal injury attorney. Use the Avvo search tool to find...

There is no requirement, other than reasonableness and the the statute of limitations to sue on a contract, as to when they can bill.

If it is a company email, they own and can use it. If they are making it as if it emails coming from you, that would be misrepresentation.

If you have witnesses or video, your case would be bolstered. You may need to hire a criminal defense attorney who can then see what evidence may...

Your statements are all defenses to their claim to be entitled to payment, subject to proof of your points. You might be best providing this...

No. But sub service is available https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CCP&sectionNum=415.20

You have to make sure you are signing in your representative capacity, only.

Consult with an experienced attorney in your state. Use the Avvo search tool to find an attorney. https://www.avvo.com/find-a-lawyer.

Yes, if the agreement with them so provides.