I am a survior of such an happening. In the longrun SHE is the one that will look bad and when parents realize how much stream she is blowing, they will test you but they will realize how full of it she is.
Stay cool, stay calm, vent ONLY to close friends and at home (and here ) but when the smoke clears, you will be standing tall .
I agree with COPrez, Taking the high ground is the way to go. I would keep aware of the ex-prez group's actions, keep everything in your group accountable and HAVE FUN WITH THE JOB. Give it a couple months and all of a sudden you're on the "runaway train" to happiness and she's out of gas. Few people follow a no-class act (as you described the ex-prez) for very long - they get on with their lives!!
Kill them with kindness! You have made many positive changes to the school and I'm sure many parents have noticed. Even though you don't see eye-to-eye with this person, you also know that there's a heart (somewhere) in that person that wants to do good things for the children. That's what it's all about. Parents will see you are the bigger person here. Thank her (possibly publicly or via newsletter) for offering to chair the fundraiser and congratulate her on any success she has. After that, come back to this board to vent!
Last year, our group rallied for some changes in the old "clique" PTO and subsequently the president and some board members resigned. Our group stepped in and have made a lot of positive changes. Since then, the former PTO president has begun a vicious smear campaign against me (the new pres). Most recently,she volunteered to chair a large fundraiser. How naive could I be! This former pres used the fundraiser as her own personal forum and not only excluded our PTO Board Members from volunteering, but spent the majority of the fundraiser bad-mouthing me and select members of the PTO board. She's has even involved the principal and superintendent of schools! Parents are afraid to stand up to this individual for fear of retaliation. It's a runaway train! What's the best way to go on the offensive and turn things around. Help!