I have these fully upgraded 5 star heroes: Aeron, Alasie, Gregorion, Gravemaker, Horghall, Guardian Owl, Mitsuko and Aegir.
Soon they will be joined by Evelyn, Magni and Joon.
Among them, what will be the best defense team configuration?
Currently I have this one: G.Owl, Alasie, GM, Aeron and Gregorion. I really like Mitsuko and Aegir but can’t find a proper place for them.
Aeron Alasie Mitsi Aegir GM
Greg-Alasie-GM-Aeron-Joon
AI shots from left to right
Greg - dmg, crit buff
Alasie - fast, powerfull sniper
GM - best offensive tank IMO
Aeron - heal, anti debuff shield
Joon - another fast, powerfull sniper
What is the advantage of left to right, since they are nearly never all charged at the same time in defense?
Not all, right. But in many raids 2-3 enemies shoot in the same/next turn, that why. Buffs work on normal attacks too, that why this is important.
Right, will be advantageous to buff the attack before you shoot. 
You could try to do Mitsuko + Gravemaker to make them counter Ice skills. That is if you aren’t going for a rainbow defense team.
Something like this maybe:
Gregorion - Gravemaker - Aegir - Mitsuko - Alasie
You have a lot of choices though, and with the ones you have, Mitsuko is probably not worth the spot over Aeron / Joon / Magni / Evelyn.