Hire a competent CPA and do not take literal any advice you find on forums, is my first piece of advice. Now having said to ignore me, consider the below as you will-
Secondly, the whole FAA stance on commercial vs private is just within FAA's jurisdiction, that has nothing to do with the IRS tax code- completely separate. You can violate the FAA all you want and there is absolutely no way that it will impact your tax situation.
Case in point, I offer the simple predicament of failing to be airworthy/current with your airframe or your certificate- can you use that to avoid property taxes on the airplane? If a flight school loses their 141 accreditation, are they now exempt from taxes? No.
As for your fuel and maintenance expenses, none of that matters. You need milage, you are allowed to use the milage reimbursement method or fuel/expenses, whichever is greater. As I stated earlier, you need hard documentation of who/what you were doing for that flight, record of the flight (file IFR everywhere you go, for example, though FBO receipts and dates on them work just as well)
http://www.gsa.gov/portal/content/1...-radio&utm_term=mileage&utm_campaign=shortcut
$1.33 a mile, going ~200mph....yeah, i disagree with those above, it is absolutely worth it.