I could also be an equity partner with a guaranteed 15% return. 

College education is as close as you're going to get to guaranteed upward social mobility.

When I search there are some commercial NNN properties which are around or over $1MM with corporate guaranteed leases for atleast 10 years and goes up to 20 years (like Fast food Restaurant chains, Banks, medical offices etc).

I guarantee Buffett would not pay down a conventional home loan early and neither would virtually any 1%er.

First solution that comes to mind is to install a sprinkler system, but of-course there is no guarantee that tenants will use it.

I would personally recommend high cash flow because unlike appreciation, high cash flow is guaranteed.

Have them set up allotments bimonthly for each paycheck so  the money is just about guaranteed. 

I cat guarantee the Refi numbers its pure speculation based off of comps in the areas and previous sold prices

On top of considering the bonuses of mortgage paydown, tax advantages, and appreciation (even if just as a tiebreaker for appreciation since its not guaranteed), you also need to dig deeper into the second question.

Lastly do you know if DHM  requires a personal Guarantee ????