The answer the IREX wants is to divert to B immediately.
Although from memory I only had 1 or 2 questions like this when I did it so it wasn't a huge proportion of the exam.
Edit: Oh, and there is no such thing as overthinking when it comes to the IREX.
On a general note easily the most important thing to do is RFTQ, then read it again, then maybe 2 more times.
Just because you've had 10 questions all being "AWIS is unserviceable, determine minima (i.e. not subtract 100')" that doesn't mean the last one won't be "TAF is unavailable (i.e. not subtract 100' but also add 50')"
I'll always remember that stuff up because it was the only one I got wrong
As long as you've done a few practice exams the IREX isn't overly difficult. It's all in the AIP.
Good luck
