The 300-something euro LH fare is not something special though. It's the typical promo business fare (P class) that seems to have availability for your dates. A3 obviously does not.. But perhaps their cheaper business fares (A class) simply have been sold already.

You cannot judge pricing without knowing the load factors. Aegean very well may have fully booked flights for summertime when we would be able to travel again... While LH may not have. Or it may just be on those specific flights that you looked at. Keep in mind that the difference between a cheap and expensive business fare generally is only a few seats being sold.

The economy middle seat empty thing is very unlikely to happen. It happens now on the very empty flights, but, once restrictions are lifted and flights are booked full I don't believe this to be the case. I very much doubt that any European airline will actually offer such seat blocking for all economy class passengers over the summer. They will do their best to keep middle seats empty, but if they can sell more fares - they will.