Hul is the possessive form meaning their when it comes before a noun:
By contrast, hulle is usually the independent pronoun:
In everyday speech, many people do also say hulle kos, but learners will often see hul kos in more careful or standard writing.
Because hul already does the job of a determiner.
Just like English says their food, not the their food, Afrikaans says:
not
So a possessive word like hul, my, jou, ons, etc. normally replaces the article.
Yes. Klaar is a very common word and can mean things like finished, done, or ready, depending on context.
For example:
But in eet hul kos klaar, it is not simply describing a state. It is helping show that the action of eating is completed.
Yes, that is a very helpful way to think about it.
English often uses eat up to show completion. Afrikaans can use eet ... klaar in a similar way. It gives the sense that the food gets completely eaten, not just partly eaten.