1. XQD cards are a "pre-standard" implementation of what became CFE Type B. Sort of a prototype, developed mostly by Sony and Nikon. The surviving market for them is in some non-Nikon cameras and some medical and scientific instrumentation. They will probably always be more expensive, even though less capable. Since you are not using video, the XQD cards would work fine, if you happen to find a great deal on them somewhere. Otherwise, I'd target CFE-B.
2. This was answered above. The two types of cards are physically different.
3. I do not compress my raw files (D850...haven't bought my Z8 yet), but find that 100 files/GB is a good way to make a safe, quick estimate.
4. Similarity of this camera to other Nikon cameras varies. I'm working to get one because it is fairly similar to my D850 and D500. It is less similar to the D750. If you really refuse to download and scan your way through the manual, I suggest sitting down and scrolling (actively) through the menus. Use the "?" button to learn about stuff. Paul and I disagree on a number of things, but he is absolutely correct about this one.

I haven't read the entire thread but when I got my Z8's I found Hudson Henry's video's on YouTube most helpful. He goes into great detail about the Z9, which is essentially a z8 on steroids. Most of the controls are exactly the same and when they are not, he lets you know. I got my Z8's immediately when they became available, since that time Henry has released Videos about the Z8 exclusively. I am waiting until Bob Busch's book on the Z8 is released, sometime in the Spring. I found he is quite thorough in his presentation and explanations of the various functions of the camera.

If I was the owner of a Z8 my first step in learning the camera would be to download the Nikon guide for it. If a wildlife or action photographer there is no way I will miss Steve Perry's instructions on how to use the Nikon AF.
Rick is right, there are very good books available to learn the Z8 but that means extra money you have to pay. YouTube has plenty of videos from well known photographers talking about the camera. Google also offers lots of information.

In my case I never needed to buy a book to learn a camera I owned. The guide supplied by the manufacturer and my use of videos in YouTube have always been enough but that is me.