That is possible, but McDonald would surely not bother with the case if his client wouldn't share information with him. And she definitely doesn't need to waive legal privilege to do that.
I think this mystery has been pretty much solved. The defence experts couldn't explain the insulin cases at that time. Myers asked if they could go up case by case, so that he could, for example, put Hall up to answer questions on baby A but not baby F. After the judge refused, he didn't put Hall up at all.
Separately, he hoped he had discredited Evans thoroughly in cross examination. To some extent, he probably had. None of the cases relying principally on Evans's evidence got a unanimous guilty verdict. Myers's tactics weren't that puzzling and may have come quite close to succeeding.