Difference between CLDR plural rules and MediaWiki plural rules
A second ruleset is used for the languages using rules J and K. Combining both rulesets would entail increasing the number of forms for these languages by 1. As you say, this shouldn't be a great hardship, given that they only have 3 forms at present.
It sounds to me as if you would possibly like to see Mediawiki use one ruleset only for all languages and that this ruleset should be able to be used for sentences without numbers. If so, we could add a paragraph about alternative rulesets not being recommended to Plural#Alternative_ruleset.
I will try to find time to add to the discussion on CLDR, now that we have got examples of languages affected.