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 Advanced Learner’s Dictionary / OALD:

humour [British English] Pronunciation /ˈhjuːmə/ (OALD); /h j ʉ́w m ə/ (CUBE)

humorous [British English; American English] Pronunciation /ˈhjuːm(ə)rəs/ (OALD); /h j ʉ́w m ə r ə s/ (CUBE)

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)rəs/
   4. rancour, rancorous – /ˈraŋkə/ → /ˈraŋk(ə)rəs/
   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”.

Why isn’t the -ou- spelling preserved when adding -ous to humour and similar words?

As already discussed at Is "vapourise" considered incorrect, even in British English?, the -or- in words such as humorous is based on Latin spelling. The relevant suffix in Latin is spelled -or, with the stem -or-, as in the word umor, genitive singular umoris. Adjectives ending in -orosus (the Latin equivalent of -orous) are not usual in Classical Latin, but are attested from Late Latin onwards, such as saporosus, dolorosus.

Is this in some way a rule in British English when adding specific suffixes?

Yes, there are patterns or rules based on the specific suffix involved. We tend to see -or-⁠ation, -or-⁠ious, -or-ize/-or-ise, but -our-ed, -our-ing, -our-hood, -our-ship, -our-ment, as discussed and explained at Is "vapourise" considered incorrect, even in British English?.

And finally, why is it that the schwa is optional for some of these words, but not all?

Omission of schwa in these circumstances can be termed "compression", per John Wells ("clossal slebs", John Wells’s phonetic blog, Tuesday, 6 December 2011: "Most cases of compression involving schwa loss are found in the phonetic environment of a following liquid plus a WEAK vowel, as in historically hɪˈstɒrɪk(ə)li, camera ˈkæm(ə)rə, factory ˈfækt(ə)ri.")

I agree with your suspicion that it is possible for words like tumorous and vaporous to have compressed pronunciations.