I agree with keeping a copy of the email, but not forwarding it now.
So far, OP has brought an issue to the dean regarding a student being unable to participate in class, and left it for the dean to address as needed - whether the dean found that was caused by Molly acting less than desirably, or - as might have been the case - Lulu being the actual issue.
Sending dean Muffy's email would be a *new* issue brought forward - "look! Molly can't be reasonable in professional communications, on top of that other thing".
It would - to me - only be worth bringing the email to the dean if it were needed to show a pattern in a continuing issue with Molly which needed the dean to address. I think the dean *has* addressed the first issue (re Lulu) and I would not introduce the email unless Molly continued to interfere with Lulu, or other student's class participation - or if Muffy undertook continuing inappropriate communications to OP.
As for those pointing to the "she cried, so what" - I agree it is a cold statement, but it was made in the course of an unprofessional email whose overall message was "you are not a good person because you involved a higher up in something and now I look bad because I *did* behave badly - so what." It is an emotional knee jerk defensive reaction to being called out, and pretty poorly reasoned. I would not consider it evidence that Muffy enjoys making students cry, or kicking small puppies - but that she is immature, unprofessional, and defensive - though the email is inappropriate! by itself it is not something I would be asking dean/management to pursue disciplinary action for, and if I weren't asking for disciplinary action/intervention - why forward it? Dean has just *had* an example of Muffy's professional interaction skills via Lulu.