Thankyou for this part of your reply:
> That's expected: no firmware is deployed at first, the kernel complains, > d-i notices, and deploys stuff if relevant firmware packages are found, > and reload the relevant modules. This reloading is what breaks the > kernel. See upstream bug report: > > https://bugzilla.kernel.org/show_bug.cgi?id=216844 as that was an actually useful response, and explains why check-missing-firmware never got invoked.
