2 - if this was compared to 'the average worker' (whoever that is) working 5 days on, 2 days off, then there would be a maximum of 20 duty days in this 30-day cycle, which equals 5 duty hours per day. This also sounds like a reasonably non-overworked person - working 5 days in 7 for an average of 5 hours a day.
Now averages are misleading - given, but I am trying to understand the basis for comments like "... fatigue has to play a part...."
As you say: averages are misleading. Bear in mind that flight duty limits are designed with public safety in mind rather than flight crew lifestyle. What is commonly overlooked by the casual observer comparing flight duty to office duty is the cumulative effect of time zone changes, environment, and the irregular 24/7 nature (ie usually involves flying all night and weekends). Imagine how you felt afterwards last time you flew as a passenger on a 7, 10 or even 17 hour flight. Now do again the day after tomorrow. Repeat twice a week or more for the rest of the month. And year. Continue till made redundant or medical lost. Relocate to other side of world. Repeat. Now you are gaining an insight!
Flight time is calculated blocks off to blocks on, ie actually operating the aircraft.
EK limit is max 100hrs in last 28 days in order to commence a flight. Pretty much industry standard. This crew were at the upper end but theoretically ok to fly.