OP here with some updates!
First, to reply to a couple of questions: the club doesn't really have a good mechanism for kicking someone out, unless the infractions are so severe they involve criminal charges. Documenting some of the more egregious issues proved very difficult, as little was done in writing. So we don't have proof of some of her ill behaviors and can't confront her about them in a useful way.
We held the meeting. Overall, I believed it was successful. Jen made a half-hearted apology "I'm sorry *if* I upset anyone, I never would have meant anything like that..." while Tomas made, in my opinion, a more sincere apology, because he does feel that he lost it when she pushed, and he regrets that. However, though he apologized, he made it clear that there was a pattern of behavior on her part which pushed him to the limit, and that we needed to create a way to get her to back off when needed. Jen resisted this idea, and tried to manipulate the meeting by crying at the idea that people were thinking "bad things" about her (she even said at one point, "I'm sorry about crying, but it's what girls do, right? 'Cause it works!" Which I thought was a very telling statement). However, everyone at the meeting was pretty strong (while staying polite and compassionate in their wording) that she has some behavioral issues that she needs to correct. Eventually Jen was forced to agree to some measures to help with things in the future, the two of them agreed to be cordial when necessary for dealing with each other, and the meeting ended. Some of us at the meeting knew that this was only a stopgap measure and that we would have to be on the alert for future problems, but felt it was a good first step.
Since the meeting I have continued to work with her, as our positions required it. She has been not easy to deal with, continuing her demanding, procrastinating, and mooching ways, but I have endeavored to be firm about my personal boundaries while still being polite. Not always an easy job! There have been some major frustrations with a couple of projects but I am learning to shrug and let things fail if they have to. It is hard when some of these projects are dear to my heart, and affect other people.
At one point in the last month, I happened to look up Narcissistic Personality Disorder. Jen has displayed 13 out of 14 symptoms listed.
Recently, things came to a head. For a recent convention, a local group had prepared a presentation for the last month. The day before the convention, Jen, having already been notified about the local group's preparations, wrote saying that she wanted to make the presentation instead as a prerogative of the position we jointly hold - although she also said she might not make the event the next day since she was sick. At the convention, the person running it and I decided it was not worth fighting Jen on the issue, and told her (in separate conversations) that she could do it, but that we weren't happy with her about it, that she was preventing other people who didn't have her opportunities from participating fully. She pushed and pushed, seemingly wanting us to "approve" of her choice to do the presentation, and said repeatedly that she should have been asked or told before anyone else tried to prepare the presentation. We kept telling her we didn't need to keep discussing it, she was getting her way, but we were allowed to be unhappy about it. Eventually, in my conversation with her, she said one too many times that she should have been asked, and I sort of lost it, saying (not loud, but vehemently) that I shouldn't have to ask her, I hold the same office as her, the convention was in my local group and I should be able to arrange the presentation and in fact had notified her that I had done so. She backed off somewhat, then turned the conversation to, maybe she should just quit the position, this wasn't fun anymore. I calmed down, urged her not to make any quick decisions, said we should talk more at a later time when emotions were not running so high. I told her to get some food, get some rest so she could feel better. She went to find the person running the event, found her with some other people, said fairly rudely that she was leaving, she shouldn't have been talked to like that, she didn't feel well and the local group could do the presentation. Then she walked away, leaving the people who heard her just openmouthed at how discourteous she was.
Later at the convention I talked to the higher-up that Jen is attached to, telling about her upset. I touched on the situation of the day, but mainly spoke about the fact that she is saying she might quit, and asked the higher-up to possibly give some encouragement to her. I really feel some compassion for Jen - I think inside her head is not a nice place to be, she is clearly broken and dysfunctional emotionally.
Well, the day after the convention, I spoke with Jen's mentor, who already had heard about the situation from Jen. The mentor and I were completely on the same page - frustrated with the inability to get Jen to understand where she went wrong, concern for her issues but also unwilling to have those issues negatively affect dozens of people. It was a good conversation with the mentor, who is a very decent person trying hard to do right.
And the next day, I found out that Jen is saying she is quitting because she was bullied. This has stirred a whole passel of emotions in me: relief that I might not have to deal with her anymore. Sorrow that a person is so upset that they would leave a club they love. Concern that her words will negatively affect me, my friends, the projects we're working on. Incredulousness that something so small (a presentation? at one event?) grew so large. Sadness that we couldn't get through to this very talented person, to clear out the muck and make it so she could be a part of things without conflict with others.
I'm actually fairly depressed about the whole thing. My DH and some others are basically saying "Good riddance" because of all the pain and strife she has caused over the last 16 months, but I can't feel happy about this. I know she's a pain, but I also know she's *in* pain, an emotional mess, and I just feel for her as an empathetic human being. And yet, I can't say I truly want to persuade her not to go. I'm so ambivalent about it.
So that's the update. Words of wisdom would be gratefully received.