You could decide to pay the rent that was demanded during the three day notice period or you could vacate the premises before the expiration of the notice. Choosing one of these two options would avoid the possibility of having to defend yourself against an unlawful detainer (eviction) lawsuit.
I would serve a 90 day notice of termination of the lease (non-renewal). Section 8 -non-renewal requires 90 days per California law.
Post the notice to enter. If the tenant refuses to allow entry, you could serve a notice to cure or quit for violation of a lease covenant (comply with laws).
Do you have anything in writing regarding acknowledging that you had an agreement to vacate on October 15th? Do you have a fixed term lease? When did the individual die in the unit?
With that information I can better answer the question.
I would need additional information as to why it is a "nightmare" -- maybe the owner is in breach of contract for interfering with your quiet enjoyment of the premises or the landlord has breach the warranty of habitability -- however, more information is needed.
If you have the lady's agreement to discard the deposit check and application in writing, I believe that you could pursue a lawsuit against the company if they wrongfully retain possession of the deposit. Further, I would question how/why the rent posted was raised with a claim of a "misprint." Seems a bit suspicious and/or negligent.