You can't deduct it from the child support. That's up to you if you don't want to continue to pay the mortgage whether you live there or not. You still own the property. In the interests of keeping things amicable, it would be in both your best interests to come to an agreement regarding the financial settlement.
From what you have said, you are potentially looking at close to $340,000 of assets. 50/50 is $170,000 each. How much do you want to throw at it fighting it? My solicitor's bill is already at $30,000 and settlement is no where in sight. Another 5% to your ex means you get $153,000 so less $17,000 than what you would get at 50/50%. If she has your 14 year daughter with her, why does that mean you'll get screwed?
My ex has paid no child support for 18 months now and we have a 6ft 5in tall 17-year-old that eats like a horse, goes to school out of area which requires me to work around him and I rent the cheapest accommodation I can find. Meanwhile my ex stays in the beautiful family home with a tiny mortgage (similar to yours) and earns 4 times the amount I do. Sure I'm going for a minimum of 55%. He also thinks he has been screwed.