Linux supports applying microcode updates as part of the kernel boot process, so you are not dependent on the motherboard firmware to do it and you still get the microcode loaded pretty early.

Yes, see this libreboot-supported laptop as an example: https://libreboot.org/docs/hcl/x200.html

  The X200, when run without CPU microcode updates in coreboot,
  currently kernel panics if running QEMU with vt-x enabled on 2 cores for the guest.

Oops.