OP - I'm a stepmother, and whilst I've been fortunate enough not to be on the receiving end like you have, I've experienced my fiancée and father of my child be destroyed by slander and manipulation.
However, I totally agree with fuckit - I know it's not what you want to hear, you want us to support and encourage your pursuit of this woman, but fuckit really does speak sense - you need to disengage, and stop allowing this woman to affect you in this manner.
As you've said, the kids are nearly (if not already) adult age. If your children are being harassed by a stranger (which essentially she is, as there is no biological link and she is not a 'friend') then you must seek alternate advice/consequences for that.
The usage of social media is sadly a byproduct of our 'dirty laundry' society and she won't stop or be silenced, so you're best removing yourself entirely from her merry-go-round of drama because you are fuelling it with this reactive response (however justified).
Your friends and family should NOT be engaging with her either nor should they be passing on information that is going to make you feel like this - in fact, why are they in this drama at all? I would be questioning whether these were friends and family I would want around if they persisted in regaling the exes batshit drivelling - a few of mine and fiancées friends still communicate with fiancées ex (school playground) and we asked politely that their friendship with her was very much separate from their friendship with us and we didn't need to know anything about how ex conducted her life beyond whether fiancées son was happy and healthy.
Disengage and that headspace to cope will return, I promise.