Post merger, Don was a mess, hardly worked, and was a bit uncooperative so many of the partners were already frustrated with him. After the hershey meeting, most of the partners thought Don fell off the deep end and would be far more of a liability than an asset going forward--people were going to hear about that meltdown. Most of the partners (Joan, Cutler, Cooper, and maybe Ted) felt the best course of action would be to separate themselves from Don by putting him on leave, basically to try to wait Don out until he got bored/frustrated and found another job which would nullify the other partners' obligation to buy Don out.
In regards to Joan and Jaguar, that's a pretty complicated issue. Joan's equity may have been worth more post-merger, but that wealth would still be illiquid and all her eggs would be in one basket. The IPO would have instantly given her around 200k liquid (IIRC the valuation the banker gave her put her stake around 1 million and they were going to float 25% of the company). That sum would be about 1.5 million today and she would still have a significant stake in a publicly traded company.