My father's new wife was like this (notice I don't call her step mother...)
Record every unreasonable action- like saving screen snapshots as you already are
Do not meet with her one to one- you need a witness every time you talk to her.
Be prepared for every request you make/ground rule you try to set or discuss will be flouted (often outrageously)- so don;t focus your efforts here, focus your efforts on supporting your children through what could become a very distressing time for them
Don't get drawn into cat fighting- behave impeccably and record/have witnessed everything she does to hurt your children, provoke you, act unreasonably.
TBH, what I think she is really after is pushing your children out of your ExP's life. She doesn't want him to have extra access- she wants him to herself. But she wants you to be the bad guy in that process , not her. She will be very supportive to him, she will set the scene with him so you look unreasonable (hence the co-parenting link etc) whilst doing her utmost to make your life unbearable, and then any time your respond/complain, bingo, you look like unreasonable ex to their Dad, and if the relationship breaks down/turns sour, it all your/DCs fault.
She is trying to make the relationship so difficult for you and your children that you withdraw and they get pushed into the background. She will try being so provocative that you complain or get access conditions altered, and little by little DCs will get pushed out.
I'm a feminist, but god, some women can be evil.