"To which place did you go?"
Translation:Al kiu ejo vi iris?
Only with yes-no questions. You can't use it if there is another question word (in this case, kiu).
Yes and no. Theoretically, you can use the accusative on a noun without a preposition to indicate a direction. According to PMEG, in practice it's used almost exclusively with proper nouns of cities and countries and this usage is quite rare anyway. So it's not incorrect, but it's unusual.
Preposition 'stranding' is a feature of English that I haven't seen in any other language yet.
They're similar, but ejo carries more of the idea that it's a place for something.
I know this section is going over the affixes and all, but saying 'kien' is much more condensed than 'al kiu ejo', unless there's some other distinction to be made. It took kien anyways, so it's not a complaint.