Had I used contactless and my bank card was checked between KCN & SEV or as I doubled back would this have affected the contactless fare I would be charged between Rickmansworth & Knockholt?
No. With contactless revenue inspections, they are purely checking your card number to see if it's on TfL's list of blocked cards. A record is made of the fact that your card was inspected, and this is sent to TfL.
If TfL can see, when all your day's touches are reconciled by the system overnight, that you were touched in at the time of the inspection - hunky dorey. If you weren't, you'll be charged a maximum fare and get one "strike" towards the three strikes that will see your card blocked.
AFAIK, rhe location of the inspection isn't part of the data sent to TfL and indeed I would be surprised if the Oyster/contactless revenue inspection devices even know their current location when an inspection takes place between stations. Perhaps they're more sophisticated than I give them credit for!
In any event, even if TfL knew you were doubling back, that isn't against the rules for PAYG. You get charged based on where you start and end your journey. Intermediate touches/changes only make a difference if they mean you meet the criteria for a different fare than the 'default' fare (e.g. being charged for travelling via Zone 1).
There's only the default fare from Rickmansworth to Knockholt, so any intermediate touches won't make a difference to the fare - provided you remain within the maximum journey time.