The thing is despite all the folks with Dell you've worked with on this, there's no way you can MAKE them do anything. My own experience wasn't that Dell WOULDN'T help me, but that I was never able to get past lower level support to get to someone who COULD help me. Once I was able to, they stuck with me.
If you have worked with senior engineers you should have a direct email link or phone number to them. Politely email your Dell contact for an update to your case. If they do not respond, ask them point blank for a refund so that you have the request on record. If they still don't respond about all you can do is keep writing letters to upper management or contact an attorney to discuss your rights as a consumer.
What Dell (or any manufacturer) does in cases like this is set up a lab environment using the same hardware and try to duplicate the problem. This is how they determine if the problem lies in a design flaw or an OS limitation. They co-ordinate their testing with their Microsoft counterparts if it comes down to the OS. Of course I have no way of knowing, but possibly that is the holdup.
When you purchased from Dell, you could have returned the system within the 30 day return window without any hassle. Now, you are basically at the mercy of Dell wanting to do what's right.
You are most definitely within your rights to expect the system to run and operate properly. However, you must also understand that running 12 monitors is not exactly the norm. Yes, I know these monitors work on your older system, but it's like comparing apples and oranges. Newer systems are designed entirely differently than old ones. Their motherboards, power distribution, etc are not the same. Video cards are not the same. Even if it is not a hardware issue, there may be some limitation in Windows 11 as to the number of video outputs one can use. Having 12 video outputs doesn't necessarily mean one can run all 12 at one time if the OS limits it.