To recap: comparative and superlatives can take two forms. If we have a hypothetical adjective "zonky", it can either take the 1) inflected form (zonky / zonkier, zonkiest) or 2) the periphrastic form (zonky, more zonky, most zonky). Some adjectives take form 1, some take form 2, and some can take either. The difficulty is which form to choose. For a particular adjective, you find that different grammatical authorities, and even different individual speakers, disagree strongly.
A case in point: I ran recently into a query about the situation for the adjective "stupid". To me at least, the quickest way to answer this was a corpus check using Google Books Ngram Viewer. Here are the results:
"stupider"/"more stupid" - UK English / US English
"stupidest" / "most stupid" - UK English / US English
The data shows them to be long-standing co-existing forms. Within the time slot covered, in both UK and US English, "stupider" has always been a less common form than "more stupid", though in US English, over the past 30 years, the two have come to be used about equally. "Stupidest" and "most stupid", in contrast, have been used about equally since 1860, though again in US English, over the past few decades the inflected form "stupidest" has risen to be the dominant one.
Here, furthermore, are examples of "stupider" used by writers of impeccable credentials:
- The stupider I am, myself, the stupider I think books ... - John Ruskin
- You have liked many a stupider person - Jane Austen
- His father was certainly the stupider of my two brothers ... - Algernon Swinburne
- I have seen a duke (No matter which) turn politician stupider - Lord Byron
- ... only in my obstinate persistence, like a man trying to dig though up to the neck in mud, grew stupider and stupider! - Thomas Carlyle
- the stupider and clumsier they grew ; till at last they were past all cure - Charles Kingsley
- Do not do an unjust thing now, and imagine Kanaka legislatures do stupider things than other similar bodies. - Mark Twain
- I think that most modern people are much stupider than those in the age of my father - GK Chesterton
- both your husbands have run away from you to much plainer and stupider women - George Bernard Shaw
- It was also found that the stupider animals, such as the sheep, hens, and ducks, were unable to learn the Seven Commandments by heart. - George Orwell
- I might have made a tolerably good buffoon, if I were a little stupider and a little more high-spirited - Aldous Huxley
- How much stupider! Michaelis knew at once he had made an impression on her. - DH Lawrence
- On the other hand he is in many respects stupider than the animals - CG Jung
- Besides, Lord Lambeth's no stupider than any one else - Henry James
- But at the next lower level he is replaced by a stupider homunculus ... - John R Searle
finding a word in a modern dictionary doesn’t make it legitimate, standard English... and irrelevant appeal to logic and comparison:
Stupid is just like lucid (same -id ending) ... In fact, in general, -id words use more and most instead of –er and –est.This may well be the case for "-id" words in general, and Google Books Ngram Viewer shows that for "lucid", the inflected forms are very rare (see "lucider" / "more lucid" ... "lucidest" / "most lucid"). However, there's no reason, beyond wanting language to be tidy, why "stupid" should behave the same way as "lucid", and the observed print usage shows that it doesn't now and never has.
The reality is that the choice between inflectional and periphrastic comparatives is a very complex matter, and research has shown that it varies not only with time and between speakers, but also with register (situation and subject matter). Papers and books have been written about it: see, for instance, Competing forms of adjective comparison in modern English: What could be more quicker and easier and more effective? by Merja Kytö and Suzanne Romaine; and English adjective comparison: a historical perspective by Victorina González-Díaz. The idea that such a subtlety can be summed up with a blanket statement that "form X is the only correct one" is quite bizarre. As Gabe Doyle at Motivated Grammar puts it - see You don’t really get language, do you? - it springs from a centrally mistaken and counterfactual assumption that there is One Right Way for any usage situation.