The filter shouldn't be an issue being you removed the DE grid and still had the same high pressure. Sure sounds like some kind of restriction downstream from the filter. Any restrictions before the pump should lower your pressure at the filter, not raise it. You mentioned an inline chlorinator. Some, like Hayward, restrict the opening down to close to 1/2" where the flow goes through the chlorinator. I had a check valve that used to break about every 2 years and get stuck right there. I since replaced the check valve with a Jandy and replaced the chlorinator with a Pentair 320 which has full flow through the chlorinator. How hard is it to bypass your chlorinator to eliminate it as a potential problem? After that it's all just pipe to your pool. How many returns?