I think testing a candidate in an interview situation is not a very good way to select a candidate, unless this is how you design and create products in your company (I hope not). And regardless of how well they perform in the interview, there is always some degree of uncertainty for how this performance will translate into their job.
I believe that assessing a candidate's temperament and ability to solve problems is best done by simply talking to the person and working out how well they communicate with people on different levels, ranging from something more technical to high level/general conversation. Because this is the skill they will use when engaging with different users and stakeholders, not in an interview setting where the interviewer is in control of the situation.
What I would be looking for in particular is the candidate's ability to speak their mind and provide reasons for why they agree or disagree with something, because I'd want them to be able to point out things that are going wrong rather than just going with what everyone else thinks. I would also look for their ability to ask questions and show that they are thinking deeper about the information rather than just accepting it without questions, because that's not the best way to approach research and design when you are liking to be making a lot of assumptions that need to be tested.
And if a couple of the candidates turn out to have similar abilities, then it is better at that stage to assess their individual fit to the company culture, but not before you have figured out that they have the ability to get the job done. HR departments have issues letting people go once they have been hired because it indicates that they didn't pick the right candidate and they can't ask the previous ones back (even if they were still available).