OP again! I just want to clarify some things and update a little.
I've been in contact with Jen's mentor this whole time and she has been trying desperately to get Jen to behave better towards people. However, I just found out that Jen has severed that relationship. I do not know if there is anyone she will listen to even a little at this point. I do have communication going with two people in positions in her home group because it potentially affects that group as well.
The positions Jen and I jointly hold are appointed jobs where we serve specific individuals in our club. Our terms will end in less than 4 months. I am not in charge of her in any way, and though I can talk to the higher-ups we serve, they pretty understandably don't want to be involved in unnecessary drama and it's been hard to know what to bring up. It's pretty clear now, and I am working on contacting them. They know that she "quit" but it's not clear what else she told them. Unfortunately it's time to involve to one extent or another. I'll let them know I'm willing to work with her if they request it (though nastygrams and discourtesies from her must stop) but that I am also prepared to handle the job solo if that is what is needed. Anything to get thru the next four months!
I understand what you all are saying about letting her take away the presentation from the local group. It wasn't the right choice, to give in because it was easier than enduring her upset. Part of the problem is that this presentation is a somewhat nebulous tradition in the club: sometimes, the people in the office that Jen and I hold handle it, sometimes a local group does it. Sometimes it doesn't happen at all. Sometimes the details are worked out far in advance, or minutes beforehand. Sometimes the person running the event handles all the details, sometimes someone in our jobs does, sometimes a totally different person, or we all just "wing it." This time, Sandy and I thought we had things covered, with the local group preparing, but Jen thought it was her prerogative, once she knew she was at the event, to do the presentation herself. And because there isn't and hasn't been a strong-set protocol to follow for planning that presentation, and she has this special position chosen by the higher-ups, it felt extremely awkward to tell her No, you can't butt in at the last minute.
If you're thinking to yourself, "this group doesn't sound very organized," well, in some ways, we're not, about some things. And while we have great leadership "within game," when it veers into interpersonal issues like these, the expertise folks have is variable. And none of us are mental health professionals, equipped to handle the level of problems she has. And the rules we have in place, the ways she could be disciplined, are very very rarely used and have huge negative meaning in our group, so people hesitate to make waves and suggest them.
If she shows up at the next event, I'll be on the thin line of Cut direct/coldly polite. If she tries to start talking about issues with me, I'll have a witness be on hand so there's no question as to what was said. I am now taking appropriate steps to ensure that Higher-Ups and Powers-that-Be know of some of her troublesome behaviors - what steps they take will be up to them. I will work with her as needed but at this point I could do almost all the work easily on my own. I will try to focus the greatest amount of my attention on some of the great projects that my group has been working on - we're supposed to be having fun! I'm going to make it fun again, durnit.