1) Call your insurance company and tell them a tenant reported an injury.

I would advise an injury attorney before bankruptcy, just in case.

I used my handyman once to replace without a permit but then I got to thinking about if for some reason the tank malfunctions and causes injury, I'd be liable and insurance wouldn't pay so for the small cost compared to leaving yourself exposed, its worth having a plumber do it and pull a permit.

., or should I add into lease that landlord NOT responsible for any injuries/death with ATV on property?

Not only are your personal assets protected in the event of a lawsuit related to injury on your property, but you will also be able to keep any rental-related debts separate from your own, thanks to the liability protection that an LLC offers.

Although his voice was a little quiet from his vocal injury, I like him as host.

In America you are one injury away from total bankruptcy unless you have health insurance--which comes from my employer.  

I talk to my GC about what’s going on and it’s another excuse, this time it’s a work related injury that is preventing him from getting the work done.

Many carriers believe that the STRs don't take care of the property as well as LTRs do, and they present more exposures for situations of liability (bodily injury or property damage to a third party).  

Even with insurance, if the injury is severe enough they can still come after you.