The neg size is 6x6, so the 25mm 'focal length' pertains to the 6x6 format.

The pinhole negs are a bit sharper in the center, but there's also a lot of light fall-off towards the edges, which emphasizes this.
It's never going to look pin sharp with the Zero, although I have seen pinhole shots that are very sharp, usually from home made pinholes.

Absolutely get the deluxe version. The cable release is a real improvement over the normal sliding shutter. The normal shutter can introduce camera shake if you're not careful or have an extremely sturdy tripod.

Framing is by feel. You can't see what you're going to record on film anyway, so you learn how to intuitively 'see' what's going to be in the frame.

If you want to, I can send you my Zero 2000 to try out. I haven't used it in a year or so.

One thing to notice is the film winding mechanism. It isn't very good, and sometimes doesn't wind the film tightly onto the uptake spool. There are metal spring blades that lean onto both spools, but I don't think they provide enough tension to either wind the film tightly, or hold the film flat. But in a way, since it's pinhole, perhaps it doesn't matter that much. They're nice to use, and the potential picture making creativity is enormous.

Good luck!