1. He hasn't long since arrived from Iraq.
This one sounds unnatural to me. I take it to mean that he has recently arrived from Iraq.
2. He reads so much by artificial light that I cannot understand why he hasn't long since gone totally blind.
... why he didn't go blind a long time ago.
... why he hasn't been blind for a long time.
3. I am probably the only person who hasn't long since forgotten the Rabbi's book.
... who did not forget the book a long time ago.
Where did you find these sentences?