Some have informed me that I must officially receive the keys and obtain evidence of the tenant's formal surrender of the property.

Tell them you purchased their father's property at the tax auction, and they will have to surrender possession of the property or you will sue all of them to enforce your rights.

Then starts the $750 a day fines and the lien.The only possible problem might be this - he says some people (the ones with a mental/hoarding disorder) will not surrender even with the fines.

Have you already decided to surrender the annuity for its cash value and intend to self-direct?  

“Whenever any person obtains possession of premises without the consent of the owner or other person having the right to give said possession, he or she shall be deemed a tenant by sufferance merely, and shall be liable to pay reasonable rent for the actual time he or she occupied the premises, and shall forthwith on demand surrender his or her said possession to the owner or person who had the right of possession before said entry, and all his or her right to possession of said premises shall terminate immediately upon said demand.”

If it appears you have already moved some place else, I will secure the property immediately so you cannot gain entry, but you can contact me within that 10 days and make an appointment to remove personal property and surrender your keys." 

My health conditions worsened severely the two months I stayed there, he also forced me to surrender the security deposit because he knew I feared being trapped in that lease despite having legal cause to break it. 

Offer 50% of their current deposit for the keys and get a written Release of Lease, date, amount in trade for $xxx at surrender of the keys.  

Depending if said mobile home was surrendered to DMV and now is an improvement on land and not movable. 

That person is trespassing if the property has been surrendered.