I’m with you on the frustration level.
There are places that you can buy a MKS GEN_L board, and there are many other solutions that will work. I’ve been considering buying a second board so that I can experiment with firmware and know that I have a working replacement. In general, the connections to the hardware are fairly generic, and I’ve also been considering buying a more capable mainboard, like one of those based on a 16 bit processor, though that would probably require changing the screen interface.
I am working on getting a good Configuration.h file, and when I’m done, I’ll post it up, so that other people don’t need to go through this crap. I’m a programmer by trade, so I figure that I’ve got a fighting shot at it. I just want to add heated build platform support, which the hardware should handle.
I will say that most likely what you’re missing with your Configuration.h file is the line for the display type. The display that my kit shipped with is the RepRap Discount Controller. Sometimes listed as a 2004 (20 columns by 04 lines display) This might help you. Chances are that you have don no damage to the controller by uploading your firmware, but it needs to be set right in order to utilize the display. I believe from the documentation that you configure the firmware as an Arduino 2560 + RAMPS 1.3/1.4 board. That with the RepRap Discount Controller should get you something that at least gives you the basics.
You will have to set the bed dimensions (250mm by 220mm) and configure it to use the X_MAX, Y_MAX and Z_MIN endstops. IIRC, you want to use use inverted logic (positive trigger) for the endstops, but I could be wrong there. I don’t know about motor control.
I have an actual spare Arduino/RAMPS combination, and if I don’t buy a spare MKS GEN_L board, I’ll use that to develop the firmware. I haven’t done so yet because a 24V power supply for this is something of a bother.