I'd not touched the kernel after first install and set bios to legacy when that was the only workaround to a losing all screen output bug when dropping out of X.

After update I was still on the old kernel with a severely broken cli functionality...No X, no networks...There was no change in the boot ordering but I could not access it because vfat was also not readable so I used a usb-boot disk made during first install and DL'd the new ISO from another machine, used that to reinstall still under legacy and now have

mounting /dev/initramfs on /mnt failed: no such device

Followed by no /sbin/init (which exists) and a # prompt freeze/ using legacy or uefi.

I'm attempting to chroot from the usb-boot and running eliloconfig tells me there is no mtab to work with. Bootloaders and chroot are not my strongsuit... Is chroot /dev/sda2 /mnt enough to get elilo working from the right place?

I've also never had anything in /boot/efi/ which I see mentioned as being where these things should be but they never were on first install.

if you used latest current, then it will be same as 15.0 just before Pat pushed the latest update in -current

Possibly you updated the kernel-* packages, and you need to create a new initrd containing kernel modules for the same version of the kernel that you are booting from.
This should make you able to boot from the installation media: https://docs.slackware.com/howtos:sl...oot_from_media
Once you are running, check the contents of the /boot directory for the installed kernel version.
Then, ensure a proper /boot/initrd.gz exists, or regenerate it using mkinitrd: https://docs.slackware.com/howtos:sl...ystemupgrade?s[]=mkinitrd (look for mkinitrd about at the middle of the page)
Then, depending on the bootloader you want to use (lilo, elilo, grub, ...) you need to inform it of the correct location of your kernel and initrd files. At the same site you can find documentation about the different bootloaders.

Slackware doesn't auto update initrd.gz or bootloader when the kernel is upgraded. With elilo will need copy new kernel and regenerated initrd.gz to /boot/efi/EFI/Slackware and maybe edit /boot/efi/EFi/Slackware/elilo.conf depending on how it is set up or rerun eliloconfig.

No,
mount root partition to /mnt, mount efi partition to /mnt/boot/efi, mount --bind /dev /proc /sys /run /run to /mnt/dev /mnt/proc /mnt/sys /mnt/run respectively before chooting into system.

It wasn't current as much as uncurrent....I installed it a year ago as current and only added a few SBo's.

It was confusing but I got there a different way...I noticed from the usb-emergency-boot disk I made on first install that EFI/Slackware did not exist but I could boot into the system with a new install-usb and it showed them there so I copied them from the usb-OS to a temp folder then into the efi/EFI with the e-boot. Then I noticed the first error gone in the boot sequence but a missing fstab one was there. fstab was having the same thing (What I thought existed only did in the install-usb boot) and realized I might have messed up that step in the setup menu so essentially reinstalled exactly what I'd done before but double-checked everything and this time elilo configured itself correctly and I now have a working system.

Still not sure why elilo failed in the first instance to set the new kernel but I'm not touching it again unless quite necessary.

Cheers.