Instead of Mana becoming an akuma he was killed by a stray devil. Allen's cursed eye lets him see the soul of maddened stray devils aka those that become monsters. Trained by Cross Gremory before being told to go to Koah to become Rias's rook (slightly before Issei becomes a devil). Cross is Sirzechs twin brother but travels the world and doesnt really contact anyone. Black Order doesnt exsist cause Akumas dont exsist here. (you can use black order characters though) Noah are a faction in Khoas Birgade

Allen combat outfit his exorcist coat with out any holy items

Allen's Sacred Gear is slightly holy but doesnt effect him (he normally wears a glove over it and have it wrapped in bandages so it wouldnt effect others when he touch them unless its his bere gear)

(cause itll be funny) Allen's gear holy aura and his coat makes everyone mistake him for an exorcist and being alone a stray one so he is attacked and Sirzechs has Cross's letter toRias about Allen

Timcanpy is Cross's familiar

Issei takes 6 pawns 2 are mutations so Rias can have pieces for Roseweiss

Allen's eye can see super natural beings true form but he has to focus on the person but not with stray devils

Allens gear evolution

during rate Game the claw and turn to the gun and blade thing

during peace meeting Tiki destorys his gear (goes to defend Koneko even with no weapon when Kuroka arrives)

fighting Loki (maybe during the fight with Kuroka depends how you want to do it) Clown Crown awakens

Balance Breaker Exorcist Sword manifests

Allen X Koneko X Road

Rias will not try to get Allen to take her as he is too much of a gentlemen

Allen still has his cheating skills

if Yuu Kanda is added he is an exorcist (Allen's rival) and he comes around during Kokobiel but you may put some history between the two of them back when Allen was with Cross

If Krory is he is Gasper's uncle