The whole thing gets cheaper and maybe easier if your (refundable) ticket home from Kiev could be re-booked from somewhere else. Can't you fly home to YHZ from another city? You final flight of the four could be a late A3 flight out of ATH to, say LHR, to connect with an AC flight home. Substitute the 19:10 to LHR instead of the 19:05 to IST. The LHR-YHZ should be cheaper than from KBP too.

I usually use an OTA but... for this kind of itinerary I would say - book with A3. They are much more flexible if there has to be a change of plan. If there is a need for re-booking, rather than just allocating you new flights as on OTA might, you can call them and choose your options. I once had a ATH-SKG-CHQ booked (because I needed the extra segment) and they suggested rebooking me on the direct flight (due to a strike). When I explained I still wanted to go via SKG, and the reasons, they were very accommodating. If it gets to the stage where you lose one of the four segments due to some delay or strike, I think it will be much easier to get it sorted out (i.e. persuade them to credit the flights originally booked) if you did everything direct with A3. Remember we ARE entering a period of strikes here in Greece right now.