They can get a head start on the cleaning that you will be asking them to do prior to your regaining possession.

You can steal real estate legally through adverse possession (personally I don't do that).

The option does not give the right of possession a seperarte lease agreement does.

Sorry to hear about your problem.DC 100c, Notice to Quit to Recover Possession of Property is the correct form.  

The Seller In Possession (C.A.R. form SIP) form is like a lease as it has a definitive termination date.

How hard will it be to get rid of the occupants once I get possession of the property?

You have an undivided interest and possession, you can rent it, only half the income can be counted toward granny's income, until they get a DPOA when they could 1. reduce her income paying management expenses or 2. sell her 1/2 interest. 

When we first took possession of our space I fell in love with a beautiful, but dilapidated 1912 Craftsman that had been unoccupied for 2 years. 

For example if ur lease expires in 3 months and they want to stay for 6 months, you can make a case of terminating the existing and doing a new 6 month lease is best for both parties.Even without a new lease, one might consider a detailed inspection of the property conditions so you know at least what's broken or damaged at the time of your taking possession.

Unlike my parents, I don't have a bomb shelter, a 6 month supply of food from Costco, all of my wealth in precious metals or my mortgage paid off for fear that the government won't be able to take possession of a property owned "free and clear" as easily as a property that is financed.