First of all, your OVF question - and unfortunately I can't help you at all as I've been shooting with the EVF basically 100% of the time. Why, you ask? Because I'm semi addicted to both the Xpro-3's in-camera film simulations - mainly the Classic Neg but also a handful of others, including a modified Acros B&W sim, and several custom color profiles from Ritchie Roesch's cool Fuji-X Weekly website - and shooting with the EVF allows me to see the scene much more closely to how it will turn out. I'm also honestly not a fan of the way the camera AF-focuses while looking through the OVF.
But - out of curiosity - I just picked up the camera with the 18mm lens mounted - and checked the framing. I think what you said is more or less correct - the entire frame of the OVF seems to correspond to the 18mm FOV as seen through the EVF. The main difference being that one sees the lens protruding in lower frame right, which means it (the lens) cuts off the lower right-hand corner of the 'frame' which you see.
Now for my thoughts in general. When I first bought the camera, it only came with one lens, the (superb) f/1.4 35mm. I found it such a brilliant lens to use, for so many reasons, that when I finally bought the 18mm, I didn't use it that much. But, lately, I've been using it more and more, and coming to value and appreciate it more. One of the biggest things, and this is purely subjective, is that with the 18mm, the camera somehow 'feels' much more compact to me. The FOV is very, very close to several compact Ricoh's (a GRD and two different GR's) that I used to own and shoot quite a bit with, but I find myself appreciating the shooting style of the XPro3 - through its eye-level EVF - much more than I did with the Ricoh's. It makes for a more thoughtful style of photography, if that makes any sense.
To put it differently - obviously the XPro3 is never going to be a camera which you can just stick inside your jacket pocket - but with the 18mm, it feels much closer to that vibe.
I still haven't used the lens in enough situations - with enough different apertures and ISO settings - to truly have a more precise sense of it capabilities and range (something I've done more with my 35mm lens) - but it continues to please and surprise me.
One final and weird subjective comment: somehow, using the 18mm on the XPro3, it doesn't feel quite as 'wide' to me as the similar FOV on either my Ricoh's, or on a nice Nikon Coolpix A I owned briefly for awhile. I tend to favor a slightly-wide angle as my version of a 'normal' lens, on my analog film Pentaxes I shot a lot with 35mm lenses, and on some Pentax DSLR's I've had, the DA21 was my favorite slightly-wide-angle-but-still-feels-more-like-a-normal lens to me. Lately, I've started feeling that way a little about the 18mm lens. (While, conversely, the 35mm, even though it corresponds more closely to the classic nifty-fifty focal length, has a bit more of a telephoto feel to me at times.)
Hope some of these ramblings are helpful, Chris