[CUnknown]

I'd like to know how these work, too. But, let me make some guesses:

1. Does arrow fend help with either of these?

Maybe against Rain of Stones.

2. Do the stones/meteors cound as magic weapons?

I'm sure meteors do. Stones probably not.. but I believe the manual would say for sure.

3. Do Luck, Twist Fate, Mistform affect the damage?

I bet luck and twist fate would, mistform I seriously doubt (unless they are not magic weapons).

4. What are the chances a random unit is affected (not necessarily damaged) by Rain of Stones?

I thought it was 100%, isn't it?

5. How are the meteors from Gifts from the Sky spread? Do they ever hit the same spot multiple times?

From experience, they seem to be in a roughly identical pattern each time, fairly equally spaced. With their precision modifier (negative), if they were all 3 individually targeted, they'd be wildly spaced out and that doesn't seem to happen.

So, I don't think they can hit the same space.