Jono, the first shot of the tree at 18mm when converted with AccuRaw and brought into Aperture as a TIFF and then sharpened etc looks pretty good. This is on a 15" Retina MBP.
Toggling Aperture to zoom in at what it calls 100%. What I do notice is there is quite a lot of distortion control in the JPG at 18mm, the raw via AccuRaw is not corrected for distortion and the horizon curves upwards at the edges as a result, so part of the smearing in the JPG may be the distortion correction stretching the corners. The raw does show more detail in the grass, and the very distant tree branches on the horizon. My impression is that the LHS of the image may be slightly more blurred on the horizon than the RHS, but this could also be due to the nature of the subject and the rendering from raw, but at normal viewing resolutions is acceptable to my eyes.
My initial thought about it looking terribly bad compared to what I am accustomed to seeing was because I was viewing it in preview and on the retina display and that gives a different magnification at 100% to what I normally see within Aperture.
I gave up on the other picture as I couldn't tell what was in focus, which made me think OIS problem initially.
I would still try another lens, even if you could borrow a Fuji prime from someone.
Cheers