I don’t think luck comes into it. You might just be lucky enough to sit in a car for over 7 hours and leave no trace. However, if you have shot a man twice, sexually assaulted a woman in the rear seat, then proceeded to occupy the seat of the dead driver then it’s obvious you are bound to have left some traces inside the car. That is before we follow your logic regarding the clothing of Ms. Storie and also, I could add, the body of Michael Gregsten who was carried from the car.
Since there were fingerprints identified within the car then it seems unlikely that any clean up operation occurred. An experienced forensic team would be aware if such an attempt had been made.
Matching hair and fibres had been part of forensic science in the UK since at least the 1930s, so was well enough established by 1961. I would draw a different conclusion than yourself. Hair and fibres were extracted from the car but were not considered relevant to the prosecution case.