Oh btw, I just scuppered an upgrade on my old computer and had to use a rescue USB-stick. I noticed you had used various methods to get a bootable USB but wondered whether you've tried this method which I am using nowadays since I found that only one of the usb-sticks i got works as a booting medium. I tend to use this stick to boot in cases like the above and then mount an external/internal drive containing the packages needed to install/upgrade. So you can get away with a cheapish small stick.
Assuming you run in EFI-mode, then: Partition the usb stick in two and format the first ~100 Mb partition as EFI (EF00, i.e. fat32, with flags boot,esp) and the second one as ext4 (will be used as /).
Then -as root- mount the first usb-partition under say /mnt/usb (need to make that mount point with mkdir /mnt/usb; also make a /mnt/loop). Assuming you have a repository with the slackware packages in 'somewhere', then
Code:
mount -o loop /somewhere/usb-and-pxe-installers/usbboot.img /mnt/loop
Copy everything from /mnt/loop/* to /mnt/usb/ (I tend to do this in mc (midnight commander; with F5) as I can immediately see that it worked)
Unmount both /mnt/loop and /mnt/usb
Mount the second usb partition in say /mnt/hd (or whatever) and create a root folder structure on that partition (as if there is slackware installed on it): thus cd /mnt/hd; mkdir bin boot dev etc home lib lib64 lost+found media mnt opt proc root run sbin srv sys tmp usr var. Somehow this structure needs to be present for the usb-stick to work as a rescue/install-medium.
Whenever, I need to update to a newer kernel I follow the above procedure (the normally used method has made too many of the larger usb-drives I tried unbootable)