I was trying hard to edit the live.cfg in /media/user/'Kali Live',
by appending 'persistence' in the line "APPEND initrd=/live/initrd.img-4.9.0-3-amd64 boot=live components" as instructed by the link "http://www.fabbnet.net/Live_Persistent.htm",

but encounter "read-only file system" error,
kept on google till now to try to resolve it to no avail.

That is thus far what I have been working on

The link in your first post worked for me.
You don't need to do that. According to the article link above, all you need to do is create the partition with proper label and persistence.conf file within that partition and persistence will automatically be enabled upon every reboot after.

As mentioned in one of my previous posts, nothing else needs to be edited.

Thank you very much Brains.

I follows the link in my first post,
without first going through the Kali Linux install,

created sdb3, now fdisk -l shows

Device Boot Start End Sectors Size Id Type
/dev/sdb1 * 64 2097151 2097088 1024M 17 Hidd
/dev/sdb2 2097152 2098559 1408 704K 1 FAT1
/dev/sdb3 2098560 30660607 28562048 13.6G 83 Linu

this is by executing the following

end=15.4gb
read start _ < <(du -bcm ~/Downloads/kali-linux-light-2019.1a-i386.iso | tail -1); echo $start
sudo parted /dev/sdb mkpart primary $start $end

however,
parted command does inform me the recommended range for sdb3 partition may have performance issue later.

Also it warned me to also 'update' /etc/fstab

I am not sure about the above, just proceed on with

mkfs.ext3 -L persistence /dev/sdb3
e2label /dev/sdb3 persistence
mkdir -p /mnt/my_usb
mount /dev/sdb3 /mnt/my_usb
echo "/ union" > /mnt/my_usb/persistence.conf
umount /dev/sdb3

Then, I booted from USB, went into persistence session,

copy /etc direcory to /etc(copy1)

resize firefox, and set some preference settings

shutdown
and log in again to a new persistence session,

yes, changes preserved.

Many thanks,

got to catch some sleep

The "recommended range for sdb3" part may have to do with the partition not being properly aligned, most likely because the other two partitions are copies of the partitions in the Kali ISO file, which don't have to conform to disk geometry. Because the partition will only be written to, no deletes, it's not an issue.

As for the "'update' /etc/fstab", this only applies if it were to be a data partition you would want access to in which case you would include an entry for it in /etc/fstab so it's mounted automatically at boot time. It will be automatically mounted as a persistence partition on every boot, so no need to add it to /etc/fstab.