hello!
Is this a recent thing that your mare has started to do or has she always done it?
HEad tossing can be caused my several things. the first, as you suggested, can be pain in the mouth. If hse has grown hooks onthe back of her bottom jaw back molars, when she lowers her head her lower jaw can't slide forward as the teeth lock it into place. this caused discomfort. Also, if her teth are jagged, she could have some ulcers. In general horses need to have their teeth looked at at least once a year, usually every six months.
A twisted snaffle is (potentially in the wrong hands) a very severe bit. the twists are very thin and it is that surface that transmits pressure to the tongue and bars of the mouth - so a tiny pull from the rider gives a large sock of pressure to the mouth. Seeing as changing the bit has not made any improvement, why not go back to a normal snaffle or try a pelham with two sets of reins. She may be uncomfortable with the nutcracker effect of the snaffle and may have a very senstive mouth so the curb may work on her when used with the snaffle.
Head shaking can also be caused by allergies - often to pollen in the summer. Seeing as you are in Canada, I somehow doubt that it is a pollen allergy at this time of year! If you email me your fax number (or postal address) there was an article in a British riding mag called Your Horse about this two years agao which i can copy and send you.
Are you having lessons with a trainer? If she rides the hores does she still head shake? If not, it may be an evasion that your horse is using.
I hope that has helped
bye!