If you take the property subject to, ensure to get an authorization letter or a power of attorney to speak on behalf of the borrowers with all lien holders on title. 

That ended the conversation.A couple of days later the lawyers informed us they’d been authorized by the bank to pay us $250 for executing the deed.I said, “That’s great!

You can contact your local housing authority in St Louis and they generally will be happy to walk you through what is necessary.  

The tenant said they've already had someone come out and do treatment (which I was unaware of and never authorized) and said they paid.  

To not lose credibility, you need to quote what the Actual net is.

A mortgage lender may not resume acting as a mortgage lender without written authorization from the office, which authorization shall be granted if the mortgage lender provides the office with documentation which satisfies the requirements of s. 494.00611, whichever is applicable.(3) If the mortgage lender does not satisfy the net worth requirements within 120 days, the license of the mortgage lender shall be deemed to be relinquished and canceled and all servicing contracts shall be disposed of in a timely manner by the mortgage lender.

Or you could require them to authorize a credit/background check from the company of your choosing. 

Is there a form we can sign authorizing a modification and saying he’s responsible for paying to reverse it when he leaves?

As we told you before, the IRS does Not own it, so they can't sell it to you, they simply have a lien....someone would have to open probate to get the Authority to sell it to you on behalf of the estate.

I'm also an author, buy my first story won't be published for another year.