I don't know what the current situation is regarding North/South border crossing, but I've flown from Girne (Kyrenia) in Northern Cyprus to Istanbul. (If you get chance, go for an evening buffet in the Dome Hotel in Girne - just about the finest Turkish cuisine I have ever had.)
No, the siuation in Cyprus is still very tence, you will not be able to fly from Cyprus to Istanbul nor will you be able to fly from Greece to Turkey. (this was the case last year) Northern Cyprus is not reconised by any nation apart from Turkey. So you will not be able to cross the border into Northern Cyprus where flights to Istanbul are plentyful.