Here's my question...If the sinkhole is professionally "fixed" and no longer presents a threat, will our end buyer have a problem getting insurance on the property?

I attended a landlord bootcamp which I HIGHLY, 100% RECOMMEND even well experienced landlords go to, not the city run one, which I also have been to and came away thinking it should be called the "tiny bit of good info and LOTS of veiled threats if you're a future slumlord class" but its from the Apartment Assoc of SE WI (I'm a member for full disclosure) but you'll get 8 or 9 hours of PRICELESS info directly from the attorney who my usual, general practice lawyer said is hands down the one to call for any bad eviction or potentially big liability type deals and he'll go through things that the city for one, would never dream of telling you, such as how in Milwaukee Cty, our judiciary is very biased to the left side of politics as you'd guess for a generally left leaning county, but that many of these judges take those political stances right into their courtrooms and you sure as hell better be prepared for rulings that fly in the face of the current laws, especially the changes made a few years back when the GOP gained TOTAL control of state gov't in legislature as well as the governor seat and as was national news at the time, went like a machete through the law books and fixed a lot of things needing fixing, including several relatively minor aspects of landlord/tenant laws that COULD be exploited, as well as stuff that didn't make sense anymore, etc.

Google adwords is ripoff.Google lures you in with a cheap PPC rate, like 10 cents, and before you know it, they have you up to $1 or more claiming your were outbid.You have people, too, who will click on your ad repeatedly to max you out and get you off the site.Moreover, paid "ads" are heavily discounted for credibility by the cyber community which makes it even a bigger waste of money.Other than that, it's great.

I am a huge believer in sugars, so I always follow up nice, because things do get lost in cyber space.

I see the concerning concept until I prove it is no longer a threat.  

Most of them make threats but never follow through.

Secret Service Special Agent-in-Charge Parr stated, "The Secret Service actively continues to adapt our investigative techniques to progressively combat contemporary threats posed to our nation's financial infrastructure."

Even a 3% return for them is vastly superior to the 1.88% they would be getting on the ten year (with a threat of rising interest rates lowering their bond price).  

If they still refuse then go through with your threat and post on all review sites such as google, Angie’s list, etc..It may also be worthwhile to hire an attorney whom could also send a letter.

Finally, I had to take matters into my own hands to resolve it, under the threat of losing $100K+ in fees and dealing with a legal battle with the city.