The valve absolutely has to be there. Changes in pressure or volume of sand can require it to be tweaked making it dump sand one minute or not flow any the next. It is a required member for operation.
Dry air is an absolute must. I have used the small filters which help but nothing replaces a good drier. An air drier is a must if much blasting is done.The volume of air just works the air compressor heating it up and is the only way you can adequately overcome the moisture from the heated air compressor.
I also got rid of the deadman. After hours of using the blaster I found that a quick coupler screwed in the end is easier to use. I buy them by the handful. When they wear through I throw on another. The hole is slightly larger which means more volume, surface area and less air speed meaning slightly weaker. It has worked well for things I have used it for.