The problem about the shards... Soul Edge called all fragments like Cervante's and Mitsurugi's back in SCIV and it was said it was complete. Sadly not alll fragments keepers are mentioned. But if it was said Soul Edge was complete it has to mean this shards were used as well
Cervantes was a wielder of the Soul Edge as well and so was Algol in the past and Nightmware was Siegfried after all. But who knows, who will be the new wielder of Soul Ednge and the new Nightmare