Alright so here's what I understand from it all. I mean, it was clear as day to me, but I will ask you to point out if there is some problem here.
In summary: For everyone to be saved, the evil spirit must be allowed to grow stronger than Maxim WHILE the remains of Dracula are on his vicinity.
If you approach Maxim from Castle A, Juste arrives in time to save Lydie -- hence his relieved comment. She is not bitten as Maxim is holding off the evil spirit. The ending, too, corresponds to it as no mention of bite marks are made.
However, the huge problem here is that, since Lydie was saved, the chain of events leading the evil spirit to acquire its own body CANNOT be attained. Juste has the relics, but the evil spirit does not have power/presence enough to manifest using them. In other words, if the evil spirit is to be destroyed, it must be allowed to gain enough power to break free from Maxim's body. On Castle A Juste arrives too early for that to happen.
Now, if you approach Maxim from Castle B, Juste is too late and Lydie has been bitten. The evil spirit has now gained power and is THIS close to finishing the deal. Juste fights it, and as we see, now it has power enough (and incentive enough, thanks to the bracelets reawakening Maxim) to break free from Maxim's body and become Dracula Wraith. Remember the story previously? If it dies within Maxim, Maxim dies too. Now that it has its own body, it can die alone by Juste's hands.
So it does make perfect sense for one thing to be attained in one castle and not on the other. On one castle the events unfold before Lydie is bitten. On the other, they unfold after. In one the spirit has not gained enough power to manifest outside of Maxim, on the other it has. On one Maxim holds presence enough to stop the spirit from biting her, on the other the spirit has full control of his actions and can bite her just fine.