"more" is to "less" as "er" is to what?

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".