No problem about being late. I'm in no hurry.
Flushing: You are correct. The drain pipe being vertical can cause this. Usually flushing occurs by back pressure in the drain line. How far underwater in your sump is the 1-1/4" pvc drainpipe? Try 1-2 inches below the water level or at the water surface. Maybe adding a 45 to get the drainpipe on a slope.
Different question: In your overflow corner box, is it just a hole(bulkhead) for drainage? If so, does that make much splash noise too. Well, it might not make much noise now that it is backing up, but once we fix this flushing problem, the saltwater will drop the whole distance and should make splash noise. I'd recommend a "richard durso" standpipe.
Lock-Line Hissing: It's not causing bubbles to enter your tank or spray saltwater out of the lock-line, is it? Just noise, right?
Oh yeah, when I said that the lock-line didn't have to be submerged... I meant everything but the nozzle tip. =) Which you probably gathered. Also, you shouldn't have the nozzle tip too far below the water surface. In case of power outtage, you will get reverse siphon (you may have a check valve... but, I would not count on that. Best to have the nozzle barely below the water surface).