Only up to page eleven, but pretty much every point I wanted to make has already been made beautifully by AlmostAJillSandwich, AnxiousPeg and hipsterumlaut!
Am flabbergasted at all of the PP chastising OP for speaking the truth about how someone else's very rude behaviour, to OP and to other Mums, affects others. She didn't 'have a go': she responded to an invitation with an honest reason why she didn't want to accept it. PP trying to find ways for OP to have met with chatterbox and broached it more delicately are really reaching to try and dim OP's right to assertively and honestly state how chatterbox's rude behaviour was affecting her, and honestly I can't help but think if this was a forum full of men, few guys would be trying to suggest ways for a man to meet another guy he felt was rude and self absorbed, to even more gently and politely raise an issue that was quickly dealt with over text. And why should OP want to meet with the chatterbox!? I'm sure she has many better uses for her time and doesn't fancy playing cautious benevolent secret therapist to this woman, when she could be spending time with friends who actually listen to her as well as talk at her. It strikes me as an example of how women are socialised to avoid being honest about their feelings and the impact of someone's actions on them at all costs.
In the past year I've had to give and take quite direct criticism about interpersonal skills between a couple of close friends and I. It's not always easy to hear, no. But when you're being obnoxious and it's hurting others as well as your own chance to make friends, I firmly believe it's always better to find that out than be oblivious. If I was doing something rude I wasn't aware of and it was bad enough for someone to decline meeting me for coffee because it'd be unpleasant for them, damn right I'd want to know! If it was only affecting OP then sure, she could have just lied to her about why she was declining. But given they all socialise in a group and she does it to other women too, far better to be direct with her for the benefit of her and the group.
The people getting super defensive on here are perhaps people who are terrified that they display these behaviours and are projecting how they'd feel if someone called them out on it.
OP, you did the right thing. Lying would also have been fine. But you're within your rights to tell somebody how their behaviour is affecting you, I'd actually prefer social groups where people were honest with individuals bringing the group down and being rude to others so it can be identified and rectified, to groups where people are able to blunder on unaware while everyone else takes the brunt of their behaviour.
As PP have pointed out, the chatterbox may have reasons for acting this way, we can't know what they are, so all we can go on is the way it's affecting others. We don't give bullies a pass because they thought it was just lighthearted 'banter'!