IRENE KRISTAL, I, Plaintiff, v. MESOBLAST LIMITED, SILVIU ITESCU, and JOSH MUNTNER, Defendants., page-178

I don't need to read an intro on class actions. Despite your having very limited knowledge of legal processes you're presenting yourself as an authority on the matter.
You went from saying the plaintiff must be cross examined to all plaintiffs must appear as witnesses (which by the way could be 100s) to now say "the plaintiff would almost always have to give evidence". Now you acknowledge the plaintiff does not have to appear as a witness. While shareholders may appear as a witness, in this case it is most likely any witnesses presented will be experts. The opinions of some individual shareholders won't matter much, the "experts" will try to convince that the documentation MSB provided to shareholders was misleading, MSB would be arguing the opposite.
Also "how the market behaved if the proper disclosure had been made" can't be estimated is irrelevant, they can only be compensated for actual losses.