I think I'm the guy whose name escaped you. No problem - it's not the first time ;-)
I posted a while ago that the belief you cannot run in brackish water is a long standing "old wives tale" about water makers. I said one should simply not exceed the fresh water flow rate of your membrane. My post went on to assert your membrane will even be happy running in the fresh water of, say, the US Great Lakes or up the Guadiana River between Portugal and Spain.
The Dessalator manual saying only to use it in "normal ocean water" (whatever that is) is obviously written that way because their equipment uses a "Green Zone" which is calibrated for "normal" ocean water and they do not want to address any other usage scenarios for legal protection. It is similar to car makers eliminating gauges in favor of "Idiot lights".
So, yes, you are totally correct in your theory that high pressures which would give a (TOO) high production rate is (definitely) harmful to the membranes. And that is easier to do in low salinity water if you are not monitoring the flow rate, but only looking at the "Green Zone".
-- SN68 Sangaris, Tropic Isle Harbor, FL