Success! By process of elimination, I was able to pinpoint the wakeup issue to sys-usb.
With further testing, I found that removing the USB-C Thunderbolt 3 Controller from sys-usb's device list resolved the issue. Will test further to see if fiddling with Thunderbolt BIOS assist mode will help. Might a BIOS update might fix this? (The BIOS is on N23ET40W v1.15 from 2018-04-13). Is there a way to update the BIOS without a Windows 10 utility? Side notes: 1. Despite dmidecode reporting back the proper magic LENOVO and X1 Carbon magic strings, I still need to manually specify acpi_force_s3=5 into my xen.cfg, otherwise the kernel patch does not appear to work. 2. My aside about needing the LiveCD Lenovo trick was irrelevant. I can install from an install disk created with dd, so I don't know what I was doing wrong the first time. -- You received this message because you are subscribed to the Google Groups "qubes-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to qubes-users+unsubscr...@googlegroups.com. To post to this group, send email to firstname.lastname@example.org. To view this discussion on the web visit https://groups.google.com/d/msgid/qubes-users/c57c198f-d566-4b77-b1d8-74dbe051d898%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.