I hope that makes sense how I am describing this but feel free to tag me with any additional questions.

There's no way our sump pumps will be able to handle the regular water we already get PLUS the additional water from the high-rise as well.- He has a finished basement and he does not want to handle the hassle.

It does require self-employment activity, but has a lot of advantages: Compared to an IRA, Solo 401k contributions limits are roughly ten times higher.There is no custodial requirement for the 401k.You don't need the additional expense and administration of an LLC to have checkbook control.There is a built in-Roth component whereas IRAs are either traditional or Roth, not both.A spouse can also participate in the same Solo 401k plan.The Solo 401k has additional tax benefits over an IRA when investing into real estate using leverage.The penalties for prohibited transactions are less severe, though it's best not to utilize this benefit :)

I want to purchase several items (soaps, paper goods, additional towels, air mattresses, etc) for use by those who use the Airbnb.

You can set up each entity as a location and each property as classes, the downside will be more work (since you will need to fill out additional field for every transaction) and if you don't have a good system in place, it could end up in a mess.

What will the R-factor change be with the additional insulation?

Second, oh master architect, houses are not built out of Lego and you cannot just easily add a room or addition.

I guess I am just one of those basic lender types.put money out get a mortgage or deed of trust prom note.. lenders title policy and named additionally insured on the hazard insurance.I think your talking about an investment scheme.. :) of some sorts.. 

Also, an additional deal, if done right, could provide cash flow that he can allocate exclusively to his debt to help pay it down faster. 

To me, it's all about generating additional streams of income.