1/3 unfortunately doesn't quite work as-is. Unlike GL, Vulkan promotes extensions to core while leaving them optional in many cases.
In the case of KHR_sampler_mirror_clamp_to_edge, we have to check (and enable) VkPhysicalDeviceVulkan12Features::samplerMirrorClampToEdge. I think that may be the only case we need to worry about, though.
(Thoguh, can we even get a 1.2 device? Currently we only request 1.1.)