Hi Fridgedoctor,
In short, yes the A320 is quite close to its performance limit departing CFU on a relatively long sector such as Scotland. Also bear in mind the wind conditions (were the winds calm?) and was there a low pressure system over the island? These will both conspire to reduce the performance of the aircraft. The aircraft may have had a technical defect which would also artificially reduce its allowable takeoff weight.
Finally, what about the weather conditions in Scotland? Perhaps the crew needed to select an alternate airport further away than they'd usually do which would require more diversion fuel - again eating into the slim weight margins.
One other thing which may have messed things up is if the aircraft had an artificially reduced maximum takeoff weight (to save on enroute navigational fees). I think this is unlikely for Corfu but it can sometimes happen on longer sectors (either the airline will take a gamble if the weather conditions dictate that 90%+ of the time they'll get away with it, or there was a last minute aircraft swap).
With the fuel stop, Venice was probably chosen for fuel price and aerodrome availability plus being on the approximate route back home. You wouldn't want to do fuel a stop close to your destination as you'd be carrying extra fuel to carry the fuel if that makes sense. Halfway is the most efficient.
Hope that helps.
Nick