Very informative post, thank you, just what I was looking for.
>White cabbage?
Correct.
>Head cabbage?
More like "ball" or "coil" cabbage.
>Sour cabbage?
Correct.
>Sauerkrat is made of white cabbage. I haven't seen it made of red cabbage ever (maybe Russians do that, it sounds like a Russian thing to do). Red cabbage is head cabbage just like regular (white) cabbage. Usually here too you only say "red cabbage" but white cabbage is just "cabbage". So perhaps this is why usage of "valkokaali" confuses you there – it's not a word usually used since that's just regular cabbage – but here it is specified to disambiguate from the red variety (which I presume you'd call "punakaali", please notify me if that sounds like a word to you)
I think this sums it up well and I can now continue to attempt to cook my stew. Your translation "punakaali" is also the correct for red cabbage.
Thank you for additional information. I dipped my toe a little bit into the world of winegrapes and cultivated a german breed "Siegerrebe" once, they were preddy good.