1. Default fluxbox package in slackware show only xpm icons in menus.
run this script convert-to-xpm.sh in user home to convert png icons in ~/.cache/fbmenugen/icons/ to xpm
#/bin/bash
FILES=~/.cache/fbmenugen/icons/*
for f in $FILES
do
icon=${f%.*}
echo $icon
convert $icon.png $icon.xpm
done
sed ...

The unique thing cause my curiosity is when i start from Porteus i can use the third partition, when i boot from salix live this partition is unaccesible, the partition from the ISO of Salix are use, well the Porteus ISO is in the same partition :
Because salix live use elilo for UEFI/EFI and ...

OK. I made it quite simple.

Download this initrd.gz i have further modified. Open salixlive64-xfce-15.0.iso with iso master application and replace the initrd.gz file in boot and in EFI/BOOT folders.
Then save the new salixlive64-xfce-15.0.iso.

Edit the grub menuentry like this exactly ...

gaucho, Very good job.
I checked the links in the end and this link do not work.

boot from iso file is not supported.
init file in initrd.gz has to be updated for the iso file to be found.
Download this initrd.gz i have modified. Open salixlive64-xfce-15.0.iso with iso master application and replace the initrd.gz file in boot and in EFI/BOOT folders.
Then save the new ...

You cannot upgrade glibc, kernel-firmware, kernel packages from gslapt.
use slapt-get in a terminal
sudo slapt-get -i glibc

It is not glibc. Something else went wrong.

I did an installation from salix-live xfce 15.0 to vmware.
I upgraded the system including kernel, glibc, etc.
I did a reboot.
Then i installed the audacity (25 dependencies packages downloaded) and the vlc (35 dependencies packages downloaded). I installed multimedia codecs.
Everything is working ...

In an UEFI system you get the elilo menu. The yellow cursor is already set to Boot: cyan text box, where you can type runlevel=3 or whatever.
If you cannot type in the text box then i do not know what is going on with your keyboard.

https://people.salixos.org/djemos/salix-15.0/screenshots/salix64 ...

Obvious difference:
I get a graphical menu whereas on the laptop all is text.
This is the syslinux boot menu. It is enabled when boot to a system without UEFI/EFI.

Do I need to tweak BIOS or so?

In my laptop i have disabled secure boot and i plug in the salix live usb and press power button ...

well, I see the command line you are reffering to but whatever key I tried, I cannot get there.

Don't have to go there. Just type runlevel=3 press the buttons of keyboard.
You will see the text appeared there.