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. 

Breach of warranty of habitability - tenant's remedies(1) If there is a breach of the warranty of habitability as set forth in section 38-12-503 (2), the following provisions shall apply:(a) Upon no less than ten and no more than thirty days written notice to the landlord specifying the condition alleged to breach the warranty of habitability and giving the landlord five business days from the receipt of the written notice to remedy the breach, a tenant may terminate the rental agreement by surrendering possession of the dwelling unit.

The landlord has the right to take possession because the tenant has legally surrendered the premises.

For example Texas is 30 days from the date of surrender AND being given a forwarding address-I believe @Linda Weygant is talking about bring a small claims(or the Colorado equivilent) to which the amount does not need to be accounted for within the deposit return timeline. 

But those are a different animal in that if a buyer backs out (even surrenders their deposit) they still have the "marketing engine" running so it's easier to get another person to (in essence) pick up that contract.

Did they surrender possession by doing this or do they still technically rent because sec 8 put their $ in and we havnt gone to court/served them a summons yet?

The Amended Lease shall terminate effective as of the date that is sixty (60) days after Landlord's delivery to Tenant of the Termination Notice (the "Termination Date"), and Tenant shall surrender the Retained Premises to Landlord on or before the Termination Date in accordance with Article 21 of the Lease.

Otherwise you will miss out on invaluable lessons learned. re imo, once you assign the responsibility to someone (property manager) you surrender the most valuable part on the road to FF.