I don't know how this differs with installing Hackintosh on real hardware, but then you provide a kernel extension (kext) called FakeSMC, which emulates the SMC chip present on real Apple computers.

So in a sense, he is not using an actual SMC key, but very probably replaced it by FakeSMC. You can see that it is actually included in the repo he clones: https://github.com/SRH1605/Hackintosh-KVM/tree/master/kexts/...

Yes, but some features are unavailable such as iMessage and probably iTunes DRM, which are tied to the hardware. I was very disappointed to discover this since I was installing OSX in a VM primarily to use iMessage so that I could type my texts like a sane person instead of pecking at a touchscreen sitting next to my keyboard.

I don't recall the details offhand, but I've used High Sierra on VBox on a Debian host. Just a few VBoxManage tweaks were needed to get it to boot. I don't recall any Apple keys being required.