I pay energy costs and am wondering if I am within my rights as a renter 1)  take a portion of the energy costs out of my rent since a certified AC repair technician said the current AC system is not adequately cooling the house and 2) purchase an auxiliary AC unit (of reasonable cost ~$300, less than 1/4 of my rent)  and take that out of the monthly rent? 

Since they do not own it all they only sell you the portion they own, and they cannot warrant to you the title is clear because other folks own part of it.

Whatever he loans you as part of your joint real estate efforts should be a small portion of his retirement funds, not "a good chunk of it".

I think the daily per diem should be calculated only from the interest portion of the loan for the month...or should it be calculated from the interest portion as well as the principal?

If it is unfinished, you should be able to see everything that is going on.I would be far more concerned about the finished portion of the basement because you can't see what was done behind the walls.  

My experience is that the renter leaving typically defaults their deposit portion to the renter staying in the unit.

I should clarify, I'd owner occupy the studio portion.  

Allow me to explain; if you paid the individual with a check, credit card or money order (as long as you kept the tracking portion) you are in control.

Portion out based on square footage of each unit.

This is only a small portion of the industry which is estimated altogether to be worth over 1.83 trillion dollars.