Difference between CLDR plural rules and MediaWiki plural rules
Other systems only support one ruleset - that's the problem. And frankly I don't see any reason for using multiple rulesets for one language except for translator's convenience. As far as I know the second ruleset is always a shortcut which can be done using the first ruleset in slight longer way. Make the shortcut harder makes it even more useless and we should rather just drop it in that case.
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.