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.
And the point of language, and grammar, is to clearly communicate. A principle of Transactional Analysis is, "What I heard is what you said." This means, of course, that no matter what you think you actually said, what I thought you said is what was communicated. That is why a correct use of the language is important.
I enjoyed the excange.