It also loses its R value as the temperature decreases.

As that goes down, the interest decreases and the amount of the payment that goes to principal goes up.

I thought this would help offset the decrease in value of the property caused by any reduction in rent.

Under my understanding of your description, the cash infusion from the bank is being used to fund the project, so there will be no big post-rehab cash infusion (unless you somehow sell the property for enough to cover the refinanced loan + make some substantial profit).A possible option would be to get the property in rental shape and make the repairs over time as you accumulate cash from rental income - this would help you avoid taking on debt (and likely decreasing your monthly cashflow).

. $22,800/$600,000 = 3.8%.As you can see, if my set of assumptions is anywhere in the ballpark of reality, then you have seen a decrease in the property's Cap Rate. 

If it is renting at or above 1% of what you bought it for, it's appreciating, rents aren't decreasing, and it is cash flowing, it is likely still a good deal.

Being a multifamily and house-hacking investor myself I would naturally recommend doing your best to make a multifamily property work for you to decrease the financial burden of housing costs and accelerate your savings to funnel into further investments.

I was curious if anyone is currently using solar power as a means to advertise lower rental costs to new renters, or if the strategy of implementing solar to decrease rental costs has been done by anyone else yet?

If your remodeling to rent, it will help protect your remodel and decrease the maintenance calls out for the roof.

It could be over priced, your commission offered might be too low, you could have bad photographs, there could be something about the property that decreases it's desirability compared to other nearby properties, and many agents dont like to both with FSBOs because they tend to be very difficult and usually wont close for a variety of reasons.