Hi Andy
We used Jet2 when we went to
Icmeler in July, according to "She who must be obeyed", we booked our seats both ways. Jet2 have quite a lot of check in desks. You can still print your return boarding pass (even though you cannot use it) as it has your seat numbers on it, so when you check in, if needed you can show your return seats are booked, and they will issue the correct boarding pass. We had no problems at all.
Hope this helps.
Steve