Max wind from Campbelltown METAR in online archive was 20 mph at noon that day. That doesn't suggest turbulence downwind of Arran that would trouble a Pa28.
Unless flying very low, sea fog shouldn't affect them - the terrain would give a good VFR horizon.
If flying very low, they should be able to climb when they see the fog forming.
They'd had a full day's rest after 9 hours climbing Ben Nevis.
Waiting for the AAIB report. There's no report so far of what wreckage was like - high speed impact, or ditching.