No, not Proton, but the dxvk stuff works on Windows too. You just need to replace some .dll files
dangrousperson
Well there is d9vk and dxvk which translates directX 9, 10 and 11 to Vulkan and vkd3d which translates direct3D (directX 12) to Vulkan. So it's all Vulkan on the Linux side, Proton 'just' uses one of these to translate the game into Vulkan or passes it through if the game is already Vulkan.
I don't think there is a native (to GTA IV) Vulkan renderer in game though and therefore there isn't anything more you can or have to do on Linux.
On Windows, some older games actually get performance improvements from the translation to Vulkan.
Don't think so, they say you can have it delivered or pick it up in a store on the 5th of June, the official release date
In Germany, you can just pre order the system from the large retailers right now. Even before you can on Nintendos own website.
That is one of the options, that apparently a lot of the 3rd party devs are using. So far, all 1st party Nintendo titles are announced to include the game on the cartridge (not just a key)