Amin, you can essentially do what you want, except disable the toggle. Firstly, set the camera to your "base" preferred settings (say, aperture priority, lens wide open, whatever DR and picture style you like, AF, metering, lens FL etc), and then save it to memory bank 1. Now, always start by shooting from memory bank 1, and the camera will always start at that focal length, with those settings. You are in whatever mode you choose (aperture priority in my example above), and can do whatever you would normally do in A mode.I wish there were a menu setting to fix the focal length at 35mm equivalent or 50mm equivalent and render the zoom toggle inactive. That way I could have a 35mm prime one week and a 50mm prime the next.
If you regularly use S mode on this camera as I do to set the lowest shutter speed indoors, or float the flastest shutter/widest aperture outdoors, you can then assign S to the other two memories in that fashion, and float between all 3. In each case, your chosen FL will always be used, unless you manually change the FL.
I actually shoot with the lens ring as a zoom, because it has FL labeled (instead of just "1.3x" etc). I'm a prime shooter, and I love to be able to choose a FL on a compact, instead of interpolating the zoom ratio. In the roughly 1500 frames I've taken with the camera, I've never used the zoom rocker, on purpose or on accident. So I think the method laid out above for you would work quite well; you would be free to reassign the lens ring to whatever function you like, and you likely wouldn't accidentally bump off your chosen FL.
As to whether I would buy a fixed lens RX100, yes, I absolutely would. I'm a longtime 50mm shooter, and have shot a Leica with 50 quite happily for some time now. I could absolutely enjoy 50. I can live with any lens from 28-50 as my one and only and get photos that please me in any circumstance. Contrary to some opinions I'd love to see a fast longer FL in this camera; I think a 50/1.4 even would provide surprisingly good subject isolation. No, not razor thin DoF, but thin enough to do some really neat things.
If this camera comes out at photokina, I will preorder. I've been so happy with the RX100 as is that my XPro1 and Leica are collecting dust. It's that good!