Absolutely, IF it is the truth. Since Rebecca cannot speak for herself, I guess we have to choose who we believe is telling the truth, right?
I really don't like JS as the presumed expert on RZ's character and state of mind this late in the trial. To your point, now we're dealing with more "because Jonah said so." My guess is, if the relationship was faltering, Mary had a much better idea of how RZ was feeling right around the time of her murder.