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.
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'.