A short follow-up to the above, after the inbound flight, again with the ATR72-600 of Sky Express today. The flight was once again delayed by about 45 minutes. This delay is to be once again attributed to the delayed departure of the same aircraft from Athens.

At Corfu (our departure airport for today) there was no additional delay.
Since this time boarding was done on foot (since the aircraft parked just outside the gate area), the gate agents first called for "families with young children and all passengers with seats in rows 1-9" to board first, only to then turn away passengers travelling with young kids, who were not seated in rows 1-9 .
Anyway, this flight was as full as the previous one, yet there was enough space in the overhead bins left. This was thanks to the younger FA seated at the front, who did in fact implement the "personal item goes under your seat" rule, convincingly and yet very politely. She also had to very politely and yet very effectively intervene on a few occasions regarding seat belts not being fastened, the back of some seats not being upright and the window shades being down prior to takeoff and landing.
She even helped a passenger with a carry-on larger than what the overhead bins would allow to fit in there, by taking it off of his hands and putting it in the rear "luggage" hold of the aircraft. The ground staff at ATH even unloaded it for him and he picked up next to the stairs upon deboarding.

At this point I should probably also answer my own question from the previous post: Yes, there is space in the back of the cabin for some pieces of luggage and, in fact, once again ground crew were loading bags, that looked like carry-on ls, to that area through the rear right door/emergency exit.
So where does the main volume of luggage go to?
The dedicated space for drop-off bags is between the cockpit and the seat of the forward FA!

So, after both flights:

• Yes, those "flightradar" data are correct, they're facing delays, especially at Athens, all the time.
• They might implement common sense boarding (and other) rules, but they also might not.
• Though Aegean is certainly more likely to depart on time, which is very important if you've got connecting flights (A3 and GQ had basically the same scheduled departure time today from CFU, but the A3 flight was already landing at ATH by the time our GQ flight took off with still 1h of flight time ahead), I'd say that from outstations, like the islands, it could be worth it to book GQ for a considerably lower price (note: CFU has a fast track, to which I'd have access to as a A3 gold member, however not while flying GQ. We had to arrive much too early at CFU today in order to avoid the hordes from 11(!) other flights taking off within 60min from our own, all of them on A32s, B73s, so an estimated total of over 1500 pax, which we thankfully did, we arrived while everyone else was still at the baggage drop-off counters and went right through thanks to us flying extremely light).