Does könnyű have a plural form/is this it's plural form? It's an adjective too, right? Are we using the plural form for hidegek (cold) but not for könnyű (light) intentionally, or is there just some irregularity here in how it works?
The plural form of "könnyű" is "könnyűek".
You don't pluralize adjectives if they are directly in front of a noun. So if the sentence were "The cold chairs are very light", you'd say "A hideg székek nagyon könnyűek." <- the adjective in front of the noun remains singular, while the other one -- which is basically the predicate of the sentence -- has to be plural.
we have "easy chairs" here ( called karosszék) . the clues include easy, so this one needs fixing because " the easy chairs are very cold" was denied.
The hints just show the possible translations of a word, not taking context into consideration. Since könnyű does mean easy in many contexts, it is going to be listed, but that doesn't make "easy chairs" an appropriate translation here.