It is an FEI RULE (!) that the aids be invisible - has just been changed to 'imperceptible' - same thing just different wording. At all levels, you should not see the rider apply an aid. This of course is the ideal, but when you are on a green horse, it is **** hard! In trot, if you have an exceptionally bouncy horse, it's difficult to sit then and it is all too easy to allow your legs to move around a little. Not a good thing, but it's human nature, it happens!
As for the lower levels being boring.. how do you think the top level horses got to the top? YEARS upon YEARS in the lower levels ;) The lower levels are actually the most important, fundamental part of training a dressage horse. If you do not have the lower levels well and truly established, you cannot move up the levels because your horse will not have the strength and discipline to hold the movements and higher degrees of collection. So although the lower levels may look boring to the 'un educated' eye, they are actually the point in a horse's training where one can tell if the horse and rider combination are going to progress. it is certainly more complicated than just riding a few circles and transitions!