The magekyo grants illusion ability surpassing his current mind abilities it doesn't have a set power it's more of a multiplier and as far as the seven days and nights thing first I've heard of it while space may not be as mystical it's not easy either so comuy it that's how you spell it would still be usefull and he is still at mortal level even the gods in dxd are still considered to be slightly stronger mortals I would say and using the enslavement or illusions would help to gather information from people as well and as I said simply being older does not equate to having higher will especially if caught off guard and you saying to take something out of the equation just because you don't like it is not right either
author why do I feel, the Servan call will complement not the class that has been obtained? (which already has cester, assasin?). As a suggestion for sabers (other than arthur) lancer (valkiria), arcer (female version gilgames/athemis) berseker (tamanomae), raider (cleopatra / medusa / nobunaga)
if not scattered you are important as a minimum body as it already is
World_Atlas