For more or less similar scores my percentiles are varying in Free Mocks and Dash CATS. Which one is a more accurate one? Also with regards to CAT which mocks are close to CAT, Free ones or the Dash CATs?

Do not go with percentiles because you can't benchmark yourself against 300 or 400 people. Instead, go with percentages and see the consistency in your scores. If you are consistently scoring 90+(percentile) in free mocks and 80+ in Dashcats, then you have the potential to hit 99 percentile in actual CAT, provided you put in more effort till the very end.