Spoilers!
1: The old man was God, or a representation of God. Page 32 says "I was in the hospital a while ago and God felt so bad about my pain that he brought me a present."
2. If you are talking about 262-266 I'd say no, it didn't seem to be forced sex.
3. part1: I'm pretty sure Arie is the messenger from God described on page 172. There are several things I feel that point to this other than pg172. Pages 273-277 where Arie's mother, who would also fit into the messenger from God according to the tale on 172, tells how she had the dream she was a butterfly who saw everything the happens and mentions hearing a voice that must of been God. Then on page 31 we see Suzuki talking to a butterfly which is presumably Arie, and when Suzuki asked "Are you God?" she responds "No but I have met him before." Finally, on page 263 when Arie is conscious again she says "I can see everything that happens in the world."
3. part2: Yes they seem to be siblings. page 247 we "hear" Suzuki say "he told me I was the son of his previous wife. Not his." Also on page 6 he again states that his father is of no relation to him. Support for them being siblings comes from page 265 when Arie specifically calls him "Big brother" and on pages 276-277 we see what looks like younger versions of Arie and Suzuki playing.
That is what I'm concluded so far at least, I can't say it is the truth so don't hold me accountable