You is not an indirect object here I think, that's why we can't (I believe) say "I ask to you". In "I give you something" you is an indirect object, and that's why we can say "I give something to you". It's the same in Greek. The verb ρωτώ doesn't have an indirect object. We never say ρωτώ σε εσένα κάτι=I ask something to you, but ρωτώ εσένα κάτι=I ask you something
Generally yes, though I think there are ways of saying things in Greek that wouldn't correspond in English - as an example (for illustration, not sure if this is correct) I could ask you a favor, but this might be interpreted as "ask a favor OF you" which might take σου instead of σε. I know I came across an example like thos before but I can't remember the specifics
In Greek, the two objects are still considered to be a direct and an indirect one (Greek school grammar book: http://ebooks.edu.gr/modules/ebook/show.php/DSGYM-A112/621/4007,17983/).
From the section linked above:
Άμεσο και έμμεσο σε αιτιατική, π.χ. Ο κ. Παλατίδης μάθαινε τους φοιτητές του παλαιογραφία. Στην περίπτωση αυτή άμεσο αντικείμενο είναι αυτό που δηλώνει πρόσωπο ή προσωποποιημένο πράγμα. Αν και οι δύο αιτιατικές δηλώνουν πράγμα, έμμεσο αντικείμενο είναι αυτό που μπορεί να αντικατασταθεί με εμπρόθετο, π.χ. Φόρτωσαν το φορτηγό τούβλα (με τούβλα = έμμεσο).
Meaning: for verbs with two objects both in accusative, the direct one is that that indicates a person or some sort of personification. If no object refers to a person, the indirect object is the one that can be replaced with a phrase introduced with a preposition.
It's not, see the declension table for personal pronouns at https://en.wikipedia.org/wiki/Modern_Greek_grammar#Pronouns.
In Greek the full sentence is Ρωτάω εσένα (acc) μία ερώτηση (acc). Both objects are in the accusative. Read a relevant thread with more details at https://forum.duolingo.com/comment/26964167.