from the sounds of it you have pretty much exhausted all your options
if you have tried every tire compound and hardness and still have the problem then maybe its time to stop looking at the tires being the fault and start leaning towards what the customer can do to help prevent this from happening.
As much as we would like to he the 'fix it all' kind of techs sometimes there are some things that we just cant do alone.
Finding solutions to problems does include customer co-operation and willingness in thier ability to adapt and sometimes they need to change something thier doing to help the situation.
Is there anything they can do to help prevent this?
Could it be something the operator can do to help?

Example:
I have a customer that goes through tires on one of his lifts because of the dock plate they use to go in and out of trucks with, they keep running over the edges of the dockplate and cutting the tires up. I have evaluated the situation and notified the supervisor but they still are using the same method. I've informed him of why its happening and told him the solution being to get another type of dockplate. The ball is in his court now.