Percentage of candidates making it through to final selection will vary, depending on the standards they reach I'd presume.
CTC use PILAPT for their Wings Sponsored assessments.
Group Exercises - I have heard on the rumour mill that CTC Assessors look at these threads and change the group exercises if they are mentioned here, to others, but could range from speaking about an allocated aviation topic, to building a bridge and balancing a glass of water on it.
Hopefully others will be able to provide further information for you.
Good luck!