So glad we have you, AZlawyer for your legal expertise! WOOHOO!

I agree with Kimi_SFC, I learn ever so much from you...bless you and much gratitude always.
Since IANAL, re: motions to dismiss/demurrers. Does this mean the Judge is only ruling on the legal (actionable, and whether the conclusions/claims made by plaintiffs are explainable?) basis for the complaint vs. merits (factual persuasive evidence) of complaint?
Yes, exactly.
I find it hard to distinguish the two...because didn't you once say that any case can be considered actionable as long as there's some logic to it? E.g., A + B --> C. A happened; B happened; therefore, C???
A case can survive a motion to dismiss/demurrer if it "states a claim"--meaning that ASSUMING ALL THE FACTS ALLEGED ARE TRUE it would be OK legally for a judge/jury to rule in your favor.
When the Judge asks for facts that led the plaintiffs to believe that Dina struck Rebecca on the head, and you gave examples, "(1) Rebecca had head wounds consistent with being hit on the back of the head <I think the WDS complaint explicitly stated that Dr. Wecht mentioned the 4 concussions on Rebecca's head already so that is a fact in evidence>, (2) Dina was seen at the house <"opportunity">, (3) whatever else explains why they think those wounds were caused by Dina." Doesn't (3) by necessity have to include some type of precipitating cause aka "motive"/reason why Dina did what she did to (struck head of) Rebecca?
No, there is no need to include motive information, although of course it might be circumstantial evidence of involvement.
Do the Zahaus have to include the "means" by which they speculate Dina had struck Rebecca on the head? E.g., with the big red heavy dog bone? From my skimming of the WDS, I recall reading that the angle of wounds/lacerations on Rebecca's head indicate that something consistent with the 5 pound dog bone had been used to strike Rebecca's head...So that part seems to already be covered by WDS complaint.
There is normally no need to include the specific means of causing an injury, although if the injury were of an unusual type a judge might require an allegation in that regard.
What other facts are needed? I suppose from my non-lawyer eyes, what I see is that, anything could have led Dina to strike Rebecca on the head. When someone is in an angry, rage-a-holic state, anything can set them off. As long as you are physically present in the same room (or within the same proximity, even in another part of the house/complex, the courtyard, etc.), the enraged nut would seek you out to physically attack you, particularly if in the past they evinced a pattern of physical violence. It's almost a natural instinct for them to sooner or later blow up on you should you be so unfortunate as to be near them...
So to me, the Judge seems to seeking for specific details which nobody not even the police would have in most murders...unless they have videotapes of the murders.
However, from what you say, it seems Greer/Zahaus need only put together facts in logical order and they should be golden in having the case go forward to trial.
The Zs are not permitted to use a Complaint and the discovery process to get the information they need to state a claim. They need to have enough information to state a claim before they file the Complaint. So the judge is trying to confirm that the Zs are not "fishing" for a claim but actually have a claim. The big problem for the Zs is that they probably don't know enough IMO to state a claim against each defendant without making some guesses, which could get their lawyers in trouble ethically.
Thanks again for your legal expounders.