Thanks for the translations Azealh, I didn't expect the corridor of dreams to be an actual place, instersting designs for those new rooms.
I don't think these pictures show it to be an actual place. The girls are just laying down on some pads while the ritual takes place, but it's Danan who makes it all happen. Maybe that specific place in the tree is a key component to the ritual, but if so it hasn't been revealed yet.
I would have really liked to have seen Guts regain Cascas mind himself since they both have a shared trauma, but if Schierke and Farnese do enter Cascas nightmares, or her old memories, this may be a good way for them to learn who Casca is and what drove her mad
Guts felt like the natural choice given their history together, but in a way it's more interesting to have other people do it. They will be exposed to Casca's inner mind, see new things about her and Guts' past (which may eventually help Farnese to move on), and it could lay the foundation for them forging a stronger bond afterwards... The fact they're both familiar with out-of-body experiences is also likely to have been part of the choice. Also, we get to see their own dreams and it's already super interesting to me in and of itself. Not to mention that it shows once more why Guts needs companions, and how they are there when it really matters.
Another way to look at it is that Guts might be too close to Casca's trauma for him to be involved. He already has trouble with his own feelings. Having more neutral third parties take care of it could be for the best in that regard. Not to mention that given the nature of Casca's trauma (and her current adverse reaction to men in general), sending females inside her mind seems like the more sensitive approach. Lastly, from a storytelling perspective, Guts not getting to see Casca's inner mind first hand is probably more compelling for future developments.
What will they find there? How will they interpret it? How will they get her to "come back"? How will she feel towards Guts afterwards? It's all going to be huge.