I’m so sorry for the slow reply, I wasn’t getting most forum notifications since switching laptop, so am catching up on everything now. I recall that you contacted me via the contact form in the end, but here is my answer again.
If you send me site access I can double-check, but from reading your email it sounds like the large desktop tab is overriding your all devices setting, which it is supposed to do. Large desktop uses a min-width media query to target 1200 AND ABOVE – so that includes the full screen size you get when viewing the all devices tab.
Does that make sense? Or have I misunderstood your issue?
Also, if you find the mix of min and max width media queries confusing, you can try loading a different set via the edit media queries screen. Just make sure you don’t overwrite the existing ones if you have already applied styles via the responsive tabs (set overwrite to ‘no’)