>all the modern networks go down, commands can still be issued from anyone, to anyone using off-the-shelf hardware.
Yep, and if there's any single salient characteristic about Nuclear C3, it's redundancy. Normally the launch crews, subs, command posts, bombers, tankers, and strategic reconnaissance aircraft would get their message over secure terminals, but there are many other ways to get a valid EAM (to include via voice transmission).
There's a famous (and perhaps apocryphal) story about Jimmy Carter touring the SAC underground. He supposedly told the senior controller he wanted to talk to the boys in the launch control centers, so they set up the circuits and told the crews to STAND BY FOR A MESSAGE FROM THE PRESIDENT OF THE UNITED STATES.
This caused quite understandable consternation, as the crews were certain most of the comms networks had been taken out and the nation was somehow in the midst of a general nuclear exchange (though their detonation detection systems were not sounding). A voice message from the President never occurred again.