You are right
I think you are absolutly right. Poor milling leads to unconversion of starches that are only extracted by the higher temps of sparging. Also you are getting lower extraction efficency! The best solution is to repalce/fix your mill. Assuming at least 5% loss of extraction, figgure out how much $ you would save if you bought 5% less malt each year. I am shure this would quickly pay for the new mill!!! And beer quality would increase with less starches being sent into the boil.
IMHO you cant do a betaglucan specific test w/ iodine. If it turns black, there are alot of starches...