Why does ou change to o when adding the suffix -ous in words such as ‘humorous’?

Question

Background

I realised today that humour when made an adjective by adding the suffix -ous, loses its -ou- spelling to -o-. There are some other words which have a change in spelling, such as miracle → miraculous (presumably from Latin influence (mīrāculum), or mischief → mischievous (probably to represent the change from unvoiced to voiced pronunciation between vowels). But there is no change in pronunciation to argue the case for a change in spelling in the word humorous, as noted in the Oxford Dictionary:

humour
Pronunciation /ˈhjuːmə/

humorous
Pronunciation /ˈhjuːm(ə)rəs/

Yes, the schwa is bracketed, but the schwa is a weak vowel in any case, and it is in both cases on an unstressed syllable; one would expect it to (have the option of) be(ing) lost when concatenated with suffixes and/or words. The dictionary further comments that:

Usage
Note that although humor is the American spelling of humour, humorous is not an American form. This word is spelled the same way in both British and American English, and the spelling humourous is regarded as an error

Here is a list of some examples:

1. With schwa omittable:
   1. dolour, dolorous – /ˈdɒlə/ → /ˈdɒl(ə)rəs/
   2. flavour, flavorous – /ˈfleɪvə/ → /ˈfleɪv(ə)rəs/
   3. humour, humorous – /ˈhjuːmə/ → /ˈhjuːm(ə)rə
   4. rancour, rancorous – /ˈraŋkə/ → /ˈraŋk(ə)rəs/s/
   5. odour, odorous – /ˈəʊdə/ → /ˈəʊd(ə)rəs/
   6. rigour, rigorous – /ˈrɪɡə/ → /ˈrɪɡ(ə)rəs/
   7. savour, savorous – /ˈseɪvə/ → /ˈseɪv(ə)rəs/
   8. vigour, vigorous – /ˈvɪɡə/ → /ˈvɪɡ(ə)rəs/
2. With schwa non-omittable:
   1. clamour, clamorous – /ˈklamə/ → /ˈklamərəs/
   2. clangour, clangorous – /ˈklaŋɡə/ → /ˈklaŋɡərəs/
   3. glamour → glamorous – /ˈɡlamə/ → /ˈɡlamərəs/
   4. tumour, tumorous – /ˈtjuːmə/ → /ˈtjuːmərəs/
   5. valour, valorous – /ˈvalə/ → /ˈvalərəs/
   6. vapour, vaporous – /ˈveɪpə/ → /ˈveɪpərəs/

Questions

My main question is:
Why isn’t the -ou- spelling preserved when adding -ous to humour and similar words?

From this follows some sub-questions:

1. Is this in some way a rule in British English when adding specific suffixes? (Talking about spelling rules in English is a challenge, I know.)
2. Are there etymological reasons for this, or perhaps etymological fallacies?
3. And finally, why is it that the schwa is optional for some of these words, but not all? (My suspicion is that it simply is an error by the editors, but I look forward to learning whether there is a method behind the madness.)

Finally, my question is regarding contemporary English, not older variants, such as with e.g. ‘behavioral’ (q.v. the comments).

Notes

These questions are tangential:

1. Root pronunciation change when adding suffix
2. How come 'ou' was reduced to 'o' in the US?
3. “ou” versus “o” in spelling words like “color”/“colour”