irregular comparatives

[miniwave]

I have found in some sources that the adjective "simple" has two comparative forms:
1. more simple than (because it is a two syllable word) and
2. simpler
the same goes with adjectives like: pleasant, common, narrow and quiet.
I have also read that the comparative form of "unhappy" is "unhappier" and "unpleasant" turns to "unpleasanter"
I need to know if this information is correct because I read a lot of sources that contradict each other.
Another question is about the word "further" is it really the comparative form of the adjective "far" or is it an independent adjective that coincidently has the form of a comparative? It seems to me that because of its resemblance to "farther" it is mistakenly considered as another comparative form of far.
After all "further" always appears before a noun and it never appears with "than" after it.

10x in advance for all your enlightening and helpful answers

[MrPedantic]

Hello MW

"Farther" is a variant of "further", which is the comparative of "fore". The original comparative of "far" was "farrer", but "farther" has now displaced this.

As for the meaning, my dictionary says that "farther" is used as the comparative of "far", while "further" is used in contexts "where the notion of far is absent". However, in ordinary usage, I would say that most people do not apply this distinction; my impression is that in most contexts, most people use "further".

Of your other comparatives, I would say that "simpler", "narrower", "quieter", and "unhappier" turn up more often in standard (BrE) English than the versions with "more", while "more pleasant" and "more common" turn up more often than "pleasanter" and "commoner".

Both forms are correct, in each case; the forms with "more" are more likely to be used when emphasis is required, e.g.

1. "Did you say that ducks were as common as geese?" "No, I said that ducks were more common than geese."

It may be different in American English; maybe another poster will let us know.

All the best,

MrP