Hamako

X1 Carbon is a good choice as well.

Quite literally everything. It's my main and only OS.

Depends on the heatpipe and fan construction. Machines like the T460s can easily dissipate up to 25W while some Ultrabooks can struggle with even 15W.

I would recommend a second-hand or refurbished E7440 or E7450.

My old Clevo P150EM had a GTX675MX. Coupled with IC diamond (which I of course applied myself), temperatures stayed at 65 degrees with stock clocks and stock fan speeds. The P150EM was a pretty compact machine as well. (at least for its class) When overclocked (and keeping stock fan speeds), the GPU would reach about 70 degrees. Nowadays, machines like those don't exist anymore. The combination of 45W notebook CPU and 100W mobile GPU is now usually found in thinner and lighter machines, while machines with a chassis that is reminiscent of the P150EM tend to pack desktop internals or even higher powered GPU's, hence the high temperatures. My advice would be getting something like a P150EM and upgrading it with a GTX980M.

The P50 weighs 2.67Kg with the big battery, which is pretty good considering what you get. You can reduce the weight to 2.55Kg if you opt for the smaller 4c battery.

Check eBay. It's a good indicator.

ThinkPad P50 is also a good option. Lower end specs are quite affordable. Not everyone needs a mobile Xeon after all.

Actually no, they won't. For example the T460s can keep its full turbo clock speed when stress-tested and stays at 70°C. (see NBC review) For games however, one should always go with a Quad-core. The dual-core U CPU's aren't designed for gaming.

ThinkPads are optimized for Linux and run exceptionally well with almost any Distro. In fact, ThinkPads are THE Linux laptop. (mostly because many developers use ThinkPads for Linux) If you want to experience a similar symbiosis as macOS + Apple Hardware, then Fedora on a ThinkPads is the closest you can get. The Lenovo Outlet currently has some nice T450s machines. You might want to check those out. Make sure to get one with a FHD display. Don't get the T450.

I used a 500MHz mobile Pentium 3 until 2011.

Check the Lenovo Outlet. They have some good deals on the T450s. Make sure you choose one with a FHD display.

I switched to Linux in 2014 and have no reason to go back, so I voted for Linux in both questions.

Why don't you just buy the model that you like the most, including the dGPU, and simply not use the dGPU? That way, it will stay deactivated. The only real disadvantage is that you paid for something you are not using, but then again you said that money was no issue.

The machine you are looking for does not exist then as far as I know, as workstations tend to always have dedicated graphics.