In absolute, we're on PC, anything can be done.
Like these *.exe are publics, I do make some public comments on them.
Firstly, what you're getting with it's 1280x540, which isn't exactly 21:9 (it's 64:27 ; not very important, but I wanted to correct this).
So, in each *.exe, there are 7 adresses that contain float value stored in 4 bytes that may refere to the "ratio thing" (for the window, the in-game rendering, etc...), for the 16:9, easily noticeable thanks to used value (0x398ee33f for 0x3fe38e39 - thanks to the endian tangle - which correspond to 1.7777778, 16:9).
For PsuIlluminusOff.exe, they're the following ones :
0x881ae
0x88e0d
0x458924
0x47080c
0x470c9c
0x4855d8
0x4855e8
0x4855f8
On the PsuIlluminusOff.exe, the 1280x720 resolution hasn't been modified, and all of the above mentioned value has been edited (every 0x398ee33f have been replaced by 0x26b41740 which correspond to 0x4017b426 or 64:27 in float) and, curiously, that do the trick (in fact, only the 3rd and the 6th values need to be edited - 0x458924 & 0x4855d8 - the 3rd one is for the window ratio and the 6th one is for the rendering ratio ; that's why I do with the custom res *.exe).
The "external rendering" is this :

A 1280x540 window, with a proper rendering, excepted for the UI, which is stretched.
But in fact, the "internal rendering" is this :

It can be gotten by taking a screenshot ingame. And because of this (well... that not the only reason...), I don't judge this solution satisfying.
When I tried to obtain this, I wanted to get a proper 21:9 rendering, and with a higher resolution compared to the 120x720 offered by the game. That why the 1st thing I always did was to modify the game resolution to 1680x720... And from the moment you modify the resolution, it break the maps, because of how the game places UI element.
And what about the online *.exe, which present more interest in my eyes ?
Well... I don't intend to criticize nastily what as been done here, but the solution used is pretty poor tbh.
Sorry to stay vague, but it's preferable.
So, in case of people haven't noticed it, the online *.exe is "compressed" (thet's why his size is only 2436kb where the offfline one, in reality lighter but uncompressed, is 5910kb).
So, the PSUC.exe has been opend with a hexadecimal editor without beeing "decompressed", 0x398ee33f value has been tracked down and, two miracles, only one result exist (at 0x3c3f4), and when you replace it by 0x26b41740, it also work, which is truly miraculous (by editing like this a compressed *.exe, you don't really know what you're editing).
Sadly, doing this breaks a checksum, and the edited exe can't be uncompressed to be analyzed.
What's sure, if you do this operation properly (uncompress the exe, change on the height values the two that need to be moddified and recompress the exe), you don't get the same value at 0x3c3f4.
And unfortunately, none of this is helpful to resolve the map issues at higher resolution.