It is well-known that Avios redemption tickets carry a high surcharge on them; what you are reporting is in line with this. It is a pain, but evidently not one great enough to drive people away from the scheme. Your first two points are essentially the same, namely lack of availability. Spring is just around the corner and the promised two CW seats on every flight will have probably been taken a while ago. I suggest greater flexibility and a deeper date range and you may get lucky!