Now, for speculations, I wonder if the sword isn't showing its true power because Koutarou is still indecisive towards the girls, and in order to win this battle he'll have to fully accept his feelings for all of them.
I’m pretty sure the sword isn’t showing it’s true power because that got sealed away inside of Nalfa along with sealing her memories of being the Goddess of Dawn.
Now, for some more speculation. I’m guessing that the Gray Knight came from a world where Shiori wasn’t there to help him pick himself back up after all the girl’s had disappeared, and so he found out on his own about the Goddess of Dawn taking the girls back, without hearing about it from her herself. That would would mean his goal is probably revenge against the Goddess of Dawn (likely by killing her) for taking the girls away from him. It would also explain why he knows about the true sword of kingship: because the timeline he’s from split off after the battle with Vandarion.
Not speculation this time, but I kinda hope Nalfalaren ends up in Koutarou’s harem as a goddess as well, and not just as the Forthorthian journalist Nalfa Laren.
As of right now there is no information on when it will go up for pre-order.
Just a observation but they are probably using all their spare resources on completing the Rokujouma Kickstarter, and then will focus on releasing the other volumes after the fulfillment or when it is close to being fulfilled.