Not intended as a 'knowledgable' correction, because I really don't know - but I'd be very concerned about the solvents/carrier in WD40 softening plastic. So I'd shy away from that, myself.
My only advice would be the prolonged soak to dissolve the salt as well. I suppose it's also possible a grain of sand worked its way in there as well - either way, salt or sand crystals can with time rip up any type of o-ring seal in there and compromise it, so you definitely want it to be fixed aside from just the 'binding' issue. When soaking be careful not to jar it around a lot, since at 'surface pressure' there's not much other than just mechanical engagement holding o-ring mating surfaces together and the like. Remember as many if not more equipment floods happen in rinse tanks as at depth!
But as gert suggested, FIRST thing you should do is see if the manufacturer offers any support suggestions.