Yes. Father is dependent for tax purposes.

Joint tenancy property transfers to GF at father's death. Depending on information not provided, may be possible to proceed with Small Estate...

Mortgagee not required to change name of mortgagor. You will need to either negotiate changes or re-finance.

Cohabitation is relevant to issues of spousal support and division of community property.. I would present evidence of cohabitation to FL court.

Suing government entity is complicated. Applying for VA benefits also complicated. i have done both. You will need attorney assistance.

Yes. You may revoke POA. Best practice will include publication of revocation. Your attorney can assist with revocation, publication.

No. Sister may counsel mother to remove you as beneficiary. Possible exception if undue influence exerted by sister.

Yes. All children must be given notice. Link explains: https://www.courts.ca.gov/8865.htm

Link explains process: https://www.youtube.com/t/contact_us

Ask former attorney to file amended Abstract removing his name.