I think that "will arrive" should be accepted.
Although the sense is very similar (ie, in English, 'when do I arrive' is roughly the same as 'when will I arrive'), you probably would have to use a future tense to express 'will arrive'.
What is the difference between li and lá?
They are interchangeable, to the best of my knowledge.
remember that they are always written with an accent: "lì" and "là" :-)