Only one additional point:  if you make exception for this tenant's request, you can open yourself to all sorts of possible discrimination claims from other tenants."

It costs less than $500 to start an LLC and about $1,500 annual for LLC tax returns.It should save you a good amount of coin in the long run and provide an additional layer of liability protection. 

Keep in mind that they usually find additional damage once walls are opened up and that will cost more.

Since we have additional cash to put the required down payment on a conventional loan I'm inclined to go that way and avoid collatoralizing our primary residence but can someone help me understand the key differences between HELOC and HEIL and why I might be wrong in choosing a conventional loan over the existing equity options?

Among the benefits of this are:1) you can claim the deduction even if you do not itemize your tax deductions2) it can lower your AGI, which may make you eligible to claim additional tax deductions or credits

I think the best way to evaluate an additional investment is "Return On Investment".  

Retire , additional income , ?

There is an additional acre of land that can be subdivided and sold off that was once part of the original farmland about 200 years ago.

Also,  have a discussion prior as to your involvement if you would like to be completely passive and simply receive quarterly updates on your investment progress, or if you would like to know the challenges and why specific decisions were made along the way... latter of course takes additional time commitment. 

In addition to storage containers (which can be costly to have delivered depending upon where you are located) there are a few companies that actualy make a "portable" single storage building that are shipped flat and you tilt up the walls and have a rentalable "no building permit and no tax hit" unit ready to rent in a couple of hours.