If she and you are both happy nursing once or twice a day, don't stop! You don't have to wean her completely to get her to take formula. You may just need a different bottle (or sippy cup), and you may need to let someone else give it to her. Consider trying the Breastflow bottles, or just go straight to sippy cups! She's old enough, and may really like the handles. The Born Free sippy cups with soft spouts are great for first time sippers. :) Also, try to let dad or grandma give it to her sometimes. She may just be thinking about nursing, when she's with you.
BTW, chewing on the nipple (of a bottle) is just fine! She'll figure it out eventually, and it probably feels good on those gums. I'm betting more teeth are on the way....