Yes, eetlus is written as one word. It is a compound noun:
eet = eat
lus = desire, liking, urge
So eetlus literally has the idea of desire to eat, which is why it means appetite. Afrikaans often joins compound nouns into one word.
Yes. Het can also be an auxiliary verb in the perfect tense, for example:
Die hond het geëet = The dog ate / has eaten
But in Die hond het min eetlus, het is the main verb and simply means has.