To start I would question the amount you rent at 15%. Do you allow your renter use of your kitchen, living room and other areas of the house? Does the renter have their own bathroom or do you share? You said it was a 3 bedroom but not the number of baths. Does the renter have their own entrance or you both use the same entrance. You could be justified in allowing up to 40 to 50% rather than 15%
I would also question even reporting the additional income unless you are making a lot of profit from the rental. If you are just covering your costs it really is not worth reporting. I am assuming you have more than $150,000 AGI and are not entitled to the rent loss deduction.
Now to answer you other question about mortgage interest deduction. On schedule A it is capped as you stated. On Schedule E for rentals there is no cap.
