Learn English – “Poorer” vs. “more poor”

The paradox: "more perfect" and "less imperfect" should mean the same thing, no?

I wouldn't say that's a paradox. To my intuition, "perfect" denotes the extreme point in a continuum. Everything else in that continuum is thus "imperfect". Therefore, "more perfect" makes no sense to me, as you point out. But "less imperfect" does.

However, I have learnt that "perfect" is used in comparisons in everyday language. I am aware that some reputed dictionaries and linguists are against prescriptive advice, but since this is a Q&A site, I'd rather position myself and give positive advice. Fowler's reads on this matter:

in most circumstances perfect is used as an absolute adjective, but there are somewhat rare occasions when the speaker has in mind a near approach to such a state and a comparative adj. or the adverb very may be appropriately used with it.

In conclusion, I would try to avoid to use "perfect" in a comparison, with the only exceptions of metaphorical or approximate usages of the underlying concept of perfection.

I believe you are misunderstanding slightly. If I may rephrase the second quote:

To say that a quality is of a lower degree, you can usually EITHER add -er (one-syllable adjectives) to the end of a negative adjective or adverb, OR qualify it with less (adjectives of two ore more syllables).

To get lower degree you don't qualify the negative with 'less' you qualify the original with less. So for the adjective "pretty" the greater degree is "prettier" or "more pretty". The lower degree is "uglier" or "less pretty".

"More short" is not generally used because "short" is a one-syllable word. "Less short" is also not generally used, but if it were would mean "longer" - i.e. it has less of the property of "shortness", not less of the property of "length".