My friend Ed (an octogenarian) reminds me, "Any problem that you can fix with a checkbook really isn't much of a problem".From Napoleon Hill: "In every adverse event lies the seed of an equal or greater benefit".

I know I have to send an Adverse Action notice, but I imagine the applicant will ask what issue caused the conditional part of the approval.

Lenders are risk adversed and won't lend unless you have a track record running MF, or have a 1-5% credibility partner who has deep experience.You can gain some credibility by taking an MF boot camp.  

Lenders are much more risk adverse these days. 

I would really reconsider paying up 15k especially if it adversely affected my ability to borrow money for a while. 

Hmmm ... maybe you add to your tenant screening criteria that applicants using those mobile phone services will have some sort of adverse action.

Are there any adverse tax consequences to doing so since I’ve taken depreciation for 3 years already?

You need to have reserves for adversity.

A relatively modest - 10%-20% - reduction in the number of transactions is likely to have substantial adverse economic effects (think of all the middlemen as well as of the principals to these deals). 

Banks are risk adverse and will only give you so many before they cut you off.