On two occasions this year I've done major event shoots over multiple days using both the D3s and X-T1. The D3s is a professional workhorse well-suited to the task, except when absolute silence is required (often) and that's when I reach for the X-T1 and its silent shutter. So I ended up with a mix of shots from both cameras. I have to tweak WB, NR and sharpening separately, but thereafter I can just throw them all together. I don't see any differences of the sort that you're describing. I've printed plenty of X-T1 shots using K7 / P2 and haven't seen this issue, albeit using a different workflow.
I don't understand what you mean by "the ever changing gamma" and I haven't noticed a difference between how an image appears in Develop vs Library.
You haven't given us much more to go on to understand why you get what you do, you've simply restated the problem. There are a couple of things that come to mind, but they aren't X-T1 specific, i.e. these would also apply to a Nikon.
In the camera calibration tab in the develop module, you can select a profile. The default is the Adobe one, which is usually flat and undersaturated. So it's not a bad starting point when you need to preserve shadow detail in a high-contrast shot, but in other situations you may be better to choose another profile that mimics one of the camera's look profiles. This applies to both Nikon and Fuji.
You didn't say which colour profile you export the image from in LR or which greyscale profile. If you're exporting in sRGB for example and converting to grey gamma 2.2 (which you would, if GG22 is the default greyspace profile), then watch the histogram as you do and you'll see a significant lightening of the deep shadows. You won't see this on the screen until you soft-proof for a piezo print. This opening of the shadows is part and parcel of the piezo workflow, and won't be specific to any camera brand.
As I understand it, using the Photoshop "convert to greyscale" option uses a fairly standard weighting of
0.299 * Red + 0.587 * Green + 0.114 * Blue
which roughly equates to how the human eye seems luminosity. There are so many ways to render a B&W image from a colour one in Photoshop that I'd rarely do just that. To me, that's a significant part of getting a really good B&W print, and I'd encourage you to consider some alternatives, regardless of your issues with Fuji files.
Show us some links to these problematic images on Fuji forums.