score:2
The fact that you stow it under the seat in front of you does not make it a "personal item". A personal item is as defined in their specs, a purse, a laptop carrier, etc, something small, that is part of your everyday personal life, not a 2nd piece of luggage for travel items (daypacks float on both sides of this aspect since many travelers use them primarily for personal items like electronics, food, books, etc, while others use them as their luggage / carry on bag).
Can you carry your backpack on as a personal item in addition to your carry-on bag? If it is under the personal item size limits no problem. But as you are in violation of their size limits, that will be a judgement call by the gate agent and perhaps even the cabin crew. The fact that Peter, Paul and Mary got away with carrying an oversized personal item, does not mean you will. And what are you going to do if they say NO?
Personally I would use the laptop bag. With a multitude of flights under my belt, I find it easier to abide by the rules rather than argue with airline staff.