I'm answering this as a longtime GS leader who has always had the same (one) co-leader, who has become a great friend. I can't imagine how distressing this was for you. Adults should not save up their little grudges and annoyances and do a massive, year-end dump on another adult, which is what it sounds like she did.
So you have to be the adult and the leader here. I totally agree with Julie H. below: I would say nothing right now to Leader 2 and I would go, calmly, to Leader 3 and say, "Leader 2 called me and we had a very one-sided, very painful call. I would like to talk with you as the third adult in our troop, NOT to involve you in drama but to help ME assess whether I am off base in how I'm handling things with the troop." Keep it focused just on the troop issues raised, and on whether you can improve. Don't give a list of all the things Leader 2 said in her rather ugly vent; that only creates drama. Instead, acknowledge that Leader 2 seems to have issues with you and ask whether your own daughter is mean or not under control at times and you aren't seeing it; ask if your meeting plans are not up to snuff; etc. (I am not saying these things ARE the case but it's worth asking Leader 3 for a reality check.)
Of course, if Leader 3 is going to run immediately to Leader 2 and say, "She told me all this!" or if Leader 3 is a very retiring person who just does what she's told by 2 and you -- that might not help. So think first about how she might react and help, or not.
It seems as if this one massive phone call was Leader 2's built-up vent. She violated the key rule of any reasonable disagreement -- she piled on tons of issues that are unrelated; if she had specific issues with your GS leadership, she should not have veered off into things like your younger child's accident at her home, or your daughter's hair. So you are not off base in finding her whole call "off." I would say, if at all possible, stay put with the troop for right now; for all you know, Leader 2 has other problems in her life and this wacked-out call dumping on you was tied to some other totally unrelated stress that day.
A few questions from me as a leader to you as a leader:
Is she truly only the cookie mom or does she come to every meeting and help? Then she's more of a real co-leader than cookie parent only. It's possible that your leadership roles aren't totally clear and she feels she has more say in meetings than she actually does. Possible? Going forward next school year: Have all three of you taken the official GS training for leaders of rising Brownies? You're supposed to have it. See if you can take it together as soon as possible -- if you can do it in person somewhere (your GS Council should have listings of trainings), I would, rather than doing it separately online. Doing adult GS leader training as a team won't solve her issues or make you feel warm and fuzzy toward her, but it could help both of you be more objective - you will have to play well together for the training time, and you can bring up things like meeting planning in that environment where a trainer can answer questions. (Do not say "My co-leader hates my meeting plans....")
If you feel Leader 2 won't do training, or she backs down and says "I'm only cookie parent, I don't have to do this," do it with Leader 3 if her role is one that needs training.
I assume you're part of a GS Service Unit? If not, you really need to be. Most SUs have regular meetings you should attend (usually monthly). That is a good place to get to know other leaders of other troops. One, they can help you see that your problems aren't unique; two, you can figure out if there is another troop that would work, if you eventually move your child.
Meanwhile, do not give your daughter ANY inkling of any strife. She will absorb it much more than you realize. If the troop has summer "off" and isn't meeting, that could help calm things down. If you feel you must address this woman about it, I would stick strictly to, "While I feel I do watch my daughter and address her behaviors, if you ever feel she's not behaving like a cooperative Girl Scout in a meeting, please point it out to me then and there and I am glad to deal with it." Same for other issues, such as, "If you have ideas for meeting planning, please e-mail them to to me and I"m glad to have the input." Never bring up anything personal. At all. She did, and you know how that worked out.
If she starts calling out your child IN meetings for things you truly feel are not justified, you may have to meet with her and Leader 3 and say you need everyone to agree that each parent is in charge of her own child during meeting time. It sounds as if your child and hers play together at their house at times, so much of this venting was not actually about GS meeting behavior -- is that correct? If so, I would be too busy this summer for play dates unless they are at a neutral third place like a park. Maybe organize some Girl Scout troop play meet-ups at the park during the summer so other girls from the troop are there.