What legal actions can we pursue?

First thing to do is follow the rules for returning security deposit. Find them here: http://www.dca.ca.gov/publications/landlordbook/catenant.pdf....

If they cash the check for the full amount that would be due for a full month, does that automatically make me a month to month?

Yes, probably, depending on what your lease says. Why are you asking this question? It implies that you don't have good communication with...

Can she hold me hostage with the damages? Will I be liable for rent if she doesn't sign?

Sorry, but I only have questions in response. Why did you move out without getting roommates signature? Did you cause damages that roommate will...

Is this legal? What do I do?

Were you on a monthly agreement? If yes, landlord can end relationship on 60 days notice. The behavior you describe is unfortunate. The other...

Sure, if the lease provides for rule changes, as they frequently do. Someone would need to review the documents to be sure. Have you tried...

Should I be paying my share? or giving written 30 days notice is enough?

If you and roommates signed a lease as cotenants, each of is equally responsible for obligations under the lease.

Who is at fault in this situation?

It's not really possible to answer your question without reviewing a bunch of facts. Have you taken this issue do your broker manager? That's the...

Are they entitled to keep the majority of my deposit? What recourse do I have? What laws protect me, if any?

Well, that's the kind of thing that gives "landlord" a bad name. Review the security deposit return rules, particularly the notice section, here:...

How can I calmly and safely get my roommate out of the apartment?

You can ask her to move. Or you can ask landlord to let you out of lease. If you and roommate are co-tenants, neither of you should move out...

What is my obligation to stop illegal vending that is occurring on my property?

What a ridiculous and unfortunate situation for you. The idea that you should "build a wall" (pardon me, couldn't help it) around 16 acres is...