D.B.
Keep a record of the texts and give them to your attorney. It's called parental alienation and I'm sure it's against the divorce agreement/visitation. Don't limit her seeing him - keep a record of all the times you offer extra visitation, and how many times he calls/cancels. If the weather is bad, print a copy of "current weather conditions" or any police advisories to stay off the road - but then offer him a make-up day. Try to get a schedule that you can agree to ahead of time - if you are making up the schedule every month, he feels a loss of control. I realize you are probably trying to incorporate the kid events and activities and then work him in around that, but it contributes to his loss of control and adds to his feeling like your pawn. So he asserts his authority in negative ways.
15 year old defiance is normal and may have nothing to do with her father's influence. Make your punishments quick and to the point, but never involving him or her access to him. I agree with her stepfather that certain things have to be ignored and that you don't have to go to every fight she invites you to, but I also think kids get away with sassing and then wanting a ride to the mall. So, for example, if she says you are stupid, walk away, but then when she wants a ride, suggest that someone that stupid shouldn't be behind the wheel with a kid in the car. I'd sit her down and tell her what her responsibilities are (basically to do well in school and do just a few things around the house as part of being a good family member), and that her only job is to have the best possible time doing it. That means she gets way more privileges if she stops the arguing and the disrespect, but that it's her choice of how she wants to spend her time. If she thinks it's productive to chew you out, you don't have to agree with it or listen to it, but it doesn't make you inclined to listen when she tells you how essential it is that she has a new outfit or $20 for pizza or a sleepover. And in a gentle moment, you can let her know that the way she treats other women (e.g. you,her friends) is how she thinks a man should treat her - it says a lot about her values and her character, and she's in charge of that.