There was never going to be an acceptable resolution unless I took back some power by hiring an attorney to draft a 200-word letter.

This deal CAN still be done, but the owner in FL has to step forward and work out a resolution with the IRS, which can certainly be done, and, depending on the owner's other circumstances, the resolution may be quite favorable to the owner.  

If the tenant doesn't shows to the enforcement what will be the resolution? 

In case of disputes, explore mediation before considering legal action, and if necessary, proceed to small claims court for any remaining damages or unpaid rent.

That is correct opening another credit card will cause another inquery.Since you already have a credit card and the loan I would say keep paying your car loan and credit card on time and you should see that score rise.Even if you just charge 25 dollars in gas every little bit helps.As for the items that are on your report you will need to dispute those and get those removed.  

Jurisdiction keeps it vague, so it could be my state or city or county or community that requires it, good luck disputing that

Tenants taking ownership leads to better care and benefits both parties.Prompt Resolutions: By promptly addressing common maintenance issues, tenants can prevent minor problems from becoming serious and costly.

So, the sales contract should specify any inspections etc you want to do, including reviewing a copy of the lease and also getting estoppel forms signed so that you know what the current rental situation is in detail and that the tenant is attesting to that so there are no disputes later.

In contract disputes, everything goes back to the contract.

Another possible way to get rid of the settled collection is to send a goodwill letter to an executive of the company (not just the dispute / collection peeps) and ask for the favor of deleting the item since they no longer have a financial interest in it, you’re working to improve your financials and credit rating,  etc.