Of course, I know nothing about the candidate, but...
Something tells me that fixing your hiring is going to require a little bit more than gotchas and hotseat questions.
Why not hire based on body of work?
Frankly, an 'average skills' developer in a strong, supportive team can still be a great asset. They can do the work, but perhaps don't always know the best way to do something. However, they may be fine with being given direction. Many people I've worked with aren't good at taking directions from others - I'm not great at it myself in many situations. So the 'average skill' developer can be a blessing in many circumstances. But perhaps not on your team, which may say as much about your team dynamics as that developer.
Good teams will always find a place for people. Ironically enough, this sometimes may even mean promoting someone with poor technical skills to management, among various other drastic measures. Their technical skills may suck, but their interpersonal skills and organizational skills may be off the charts, and they'll still have enough technical background to understand what the heck they're managing. That kind of thing.
When my friend first started at Google, he commented that there seemed to be a lot of underlying impatience and discontent because Google only hired superstars, and so many superstars had to stay on the bench. Not everyone can go do the glamorous job. Various anecdotal blog posts seem to have given us that even Google hasn't been able to 100% figure it out.