Have you considered that it may have something to do with the adjectiveness or adverbiality of the word? Words that are exclusively single-use readily take the -er and -est suffixes, whereas those that perform double or triple duty as nouns and/or verbs as well (perhaps as inflected forms) are less apt to sound correct when suffixed.
Since -ly is an adverbizing suffix, most anything ending in -ly could also end in -lier or -liest. A dual-purpose word might generate ambiguity when using -er, because -er is also used to transform a noun or verb into an associated noun, much like -ing changes a verb.
"Opener" could be an adjective meaning less enclosed, or it could be a person or device that opens things.
Hot may become hotter and hottest, but heated can only become more heated, because it is already heat and -ed, and the suffixes won't combine. Old becomes older and oldest, but aged only becomes more aged. Red -> redder -> reddest; rouged -> more rouged.
Likewise, the -tic suffix does not combine. Emphatic is already an adjective form of emphasis. The -er and -est suffixes tend to attach more easily to a word that was originally a modifier rather than a noun or verb, because only a few suffixes that modify part of speech combine. I wonder if it would be better to be "most emphatic" or "most emphasized"?
The rules are complicated because English grabs vocabulary from so many other languages, and the words with germanic, romance, greek, or other etymologies don't always play well by the same rules.
Native English speakers are somehow able to apply etymologically consistent alteration rules to words based on the qualities of their sound rather than by their actual source language. That's how we got "cherry". The French word was "cerise", which was imported as "cherries". Obviously, by English grammar, a word ending in the /s/ or /z/ sound is a plural. So you chop off the /z/ to get the singular "cherry". English has sort of a Procrustean Bed approach to word acquisition. If it don't fit, we makes it fit.
I'd considered it when I had assumed it was a somewhat irregular pattern, but as I say, it turns out there is a rule and that isn't it.
You're right that '-ed' suffixes prevent '-er'/'-est' being appended even if they don't add a second syllable to their root, though in most cases '-ed' makes a word multisyllabic so forces it down the 'most' route. 'bored' or, as you say, 'rouged' are monosyllabic exceptions, though. And that's in spite of there being no awkward sound created - you could certainly say 'boreder' (it's pronounced just like 'border' or 'boarder'), but it's not a word. English is weird.
But with 'open', I don't see your point. It's interesting, in fact, that even though the word 'opener' is a valid word, in the context of a 'can opener', say, it is not a valid comparative for open. It is wrong to say 'The light through the large window makes this room much opener than that one' - you should say it makes it 'much more open' instead. But you could say it made it 'much brighter'.
Opener was an example of a potentially ambiguous word. That may be why the superlative -er is not allowed. The noun-forming -er prevails over the adjective-forming -er.
Nope, open takes 'more' because it's bisyllabic, not because it avoids a collision with the noun 'opener'. Lighter, warmer, cooler... all common enough verb-based nouns that collide with comparatives and don't take 'more' because they're monosyllables.
Your simpler explanation is certainly cleverer than mine, by far. My narrower criterion obviously has several counterexamples, so I should probably be quieter about it until I can gather more supporting evidence.
English is complicated. Let's not pretend that we know about some intentionally engineered structure in it when we're really just noticing patterns and correlations.
Since -ly is an adverbizing suffix, most anything ending in -ly could also end in -lier or -liest. A dual-purpose word might generate ambiguity when using -er, because -er is also used to transform a noun or verb into an associated noun, much like -ing changes a verb.
"Opener" could be an adjective meaning less enclosed, or it could be a person or device that opens things.
Hot may become hotter and hottest, but heated can only become more heated, because it is already heat and -ed, and the suffixes won't combine. Old becomes older and oldest, but aged only becomes more aged. Red -> redder -> reddest; rouged -> more rouged.
Likewise, the -tic suffix does not combine. Emphatic is already an adjective form of emphasis. The -er and -est suffixes tend to attach more easily to a word that was originally a modifier rather than a noun or verb, because only a few suffixes that modify part of speech combine. I wonder if it would be better to be "most emphatic" or "most emphasized"?
The rules are complicated because English grabs vocabulary from so many other languages, and the words with germanic, romance, greek, or other etymologies don't always play well by the same rules.
Native English speakers are somehow able to apply etymologically consistent alteration rules to words based on the qualities of their sound rather than by their actual source language. That's how we got "cherry". The French word was "cerise", which was imported as "cherries". Obviously, by English grammar, a word ending in the /s/ or /z/ sound is a plural. So you chop off the /z/ to get the singular "cherry". English has sort of a Procrustean Bed approach to word acquisition. If it don't fit, we makes it fit.