Can I prevent them from moving in at this point unless a payment is presented?

Especially, if it prevents property to function as intended (restricted access to mailbox, unable to use part of the driveway) it's more of a repair than the maintenance?

and that in turn is the whole reason why mortgages (deeds of trust) employ due-on-transfer acceleration clauses to prevent the collateral (property) the mortgage loan was secured against.  

Do they prevent a certain type of renter from being interested thereby causing delays in getting filled?  

I doubt if we have the hardness you recorded but yearly preventative maintenance of any system is imperative to longevity.Given the precarious attic location I would definitely review the obvious advantages of an on-demand system.I have also designed a simple fix for the 'cold water sandwiching' issue that plagues the on-demand systems with feed water temperature fluctuations common in colder climates. 

At my current brokerage, one of my trainers really emphasized how to prescreen potential buyers, and he showed me how to prevent from getting my time wasted like that.

The contract with exact specifications is much needed because it prevents the contractor to cheap out on you.

If this is true, the only thing that would prevent your tenant from moving to the other properties would be the cost and effort of moving.   

I have a clause in my lease that the tenants must keep a reasonable amount of heat to prevent damage to the property.

Has anyone found any better methods to prevent storage in the hallways?