I am wondering why there is a (restricted) copyholder at all. And why there is not a third, fourth editor instead.
You can read more about the design philosophy Scrivener adheres to in this post. Note that at this time some of the page number and section numbers may be off a little, as this was written several years ago and before the Windows manual existed.
Is there (such) a way / key (mouse combination) to always open each quick reference window fullsized?
Not like that as such, or at all at the moment. The implementation for how QR windows open is incomplete. It’s supposed to remember the last window size you used for new windows, so if you prefer a particular size or position it would just work that way (and existing windows should also remember where you put them and how big they are, from one session to the next).