I hate to disagree with the experts you reference, but just for the fun of it, how far do you go on the less clear side until you reach not clear? Can something be less perfect? Less unique? Is clear itself one of those words that cannot be modified, it either is or isn’t? Is clear a matter of opinion or interpretation? Certainly it is relative. What about opaque and transparent?
The fact of the matter is that if you must use "more" in order to properly modify the comparative descriptor and the "-er" would be awkward, then it is perfectly good grammar to use the term "more" to modify the word itself. As I said, "more clear" is perfectly good grammar if the word "clear" is used to describe being "precise" because the word "preciser" is not even an accepted word.
Certainly you can't use the words "opaquer" or "transparenter" and you are grammatically required to use More or Less to modify those terms.
In regard to your premise, when you use the word "clear" to describe absolute invisibility, then you cannot modify it at all. But when used to describe relative preciseness, it obviously can be modified to make your point a little more clear than it would otherwise be without it.