Strangely, I have quite the same problem on a mid-2013 MBAir, and that occurred immediately after the update to 15.6.0 and its EFI update… hardware problem?
To note is that the trackpad and keyboard both work fine for boot volume selection, and in hardware test.
Would someone stand up and still pretend it is an Hardware issue, maybe the low level working of the USB devices takes another route when the OS is running?