Installation failures using normal install and imager-1.7.5

It really sounds like a hardware fault so I can't advise. Try on a fresh SD card. I have never experienced a flickering screen. From what I have seen in Google a few times, its usually a power supply fault (not enough!)

This install was repeated using a fresh 32GB SD card with identical results.
Two problems: I am not looking for help on #1. Just want to post it for collective intelligence.
1) the display shows what looks like static and is similar to a problem reported in another thread where the suggested cause was the HDMI cable quality (can't find it now).
Problem: a full width horiz (static like) display distortion in a vertical band stretching from mid screen to the bottom. It does not occur spontaneously, rather only when the mouse is vertically coincident with the “five icon graphic menu” at the bottom of the screen. Moving the mouse in this vertical space generates the interference. Also, if you change to any other display format the problem is still adjacent to the now larger icon array height and the interference is still from midscreen down.

I'm a hardware guy but can't relate this more precisely with no idea of the architecture but I agree with you that it has to do with the Pi. Not at all to do with the cable or monitor.

This #2 problem I need some help on.
On the first install the display went black screen after selecting desktop “shutdown”. This time I selected “Restart”, same result (more detail above).
Has anybody encountered this ?

#1 could be the cable. I have a couple of displayport to hdmi cables on  a desktop PC . They work on windows but one of them does not work under linux.

#2 I can't help with. I wrote the docs you are following and I have never experienced a non booting pi like that. It would be interesting to stop the install at the CLI prompt and see if it fails then the same way. Then install xfce and se eif it fails then. then after adding preempt-rt and so on

If you get failures with plain vanilla stuff without preempt-rt, Perhaps log an issue with debian

I just thought, I was working with Debian 12.0 and now 12.1 is out. I wonder if the daily builds  have been broken?

Ran two more test.
3. Eliminated both Nano edits:  Would then boot to XFCE desktop after shutdown.
4. Eliminated only the second edit (disable_overscan=1  &   dtparam=audio=off)
It again boots to Xfce following shutdown.