[hannes69]

For reasons unknown,

Best start of a sentence...

But apparently madVR, with FSE and DirectX11 path, decides to use the old 59 frequency from standard Nvidia Control Panel. A resolution I have no way of selecting manually, mind.

That´s probably quite the same story I told about the 47.952 and 48.000Hz in my last post. With CRU utility you can eliminate that problem. You can erase the old standard 59 setting and replace it with your new 59 setting. It´s simple as that.

Why the heck are custom resolutions so fricking complicated?!?!?

Because it is "special stuff" for "special people"... Normal people aren´t interested in and don´t need custom resolutions. You can expect of a nerdy thing used by nerds that some time has to be invested to get that stuff running right. You want the cake, work for your cake

I would surely love for a "for dummies" guide to CRU

Hm, if you are able to create a custom resolution with a graphics vendor tool, you can do the same with CRU, there´s no difference. The terms are the same (front/back porch, sync width...) And you already detected the value of the pixel clock calculator. You just have to copy & paste the values.
In CRU: choose your device in the dropdown menu. Now on the right side you have several standard and/or detailed resolutions. Note them so if something doesn´t work as expected you have the original values. What you need are the detailed resolutions. You can delete all standard resolutions and if there are already some detailed resolutions you can delete them as well. Now fill in your detailed resolutions (the ones you used by the nvidia tool or the ones you find out with the pixel calculator tool). In my case 4 detailed resolutions are possible, actually the number we need. Take as first detailed resolution your "preferred" one, this becomes the default resolution. Reboot. Now your default resolution is active. And take a look in your OS to the choosable resolutions, they all are now there.
What can happen: Sometimes Windows or the graphics driver add a 60Hz resolution, then you normally have one resolution called 59Hz and one called 60Hz in Windows. Then you have to find out which one is your desired one. In madvr display mode switcher they are then distinguishable by their naming e.g. 1080p59 / 1080p60.
And as I already stated: A workaround that works always is to use an other multiplier. You can use 29.970Hz, 59.940Hz or 89.91Hz, whatever your TV accepts. Your TV accepts 23.976Hz so it will probably accept 29.970Hz and so you can use 1080p29 in madvr...
Good luck!