GREAT NEWS! (This post should probably be added to the first post) I've found how to remove the brightness and get widescreen on
DS VC titles on both the TV and the Gamepad as well!!
(If you aren't aware, this is Yoshi's Island DS)
The method is quite similar, but still a little different, locate your DS game and enter the same content folder as you do for the N64 games. You'll need to go one folder deeper now and enter the 0010 folder. From there, grab the configuration_cafe.json file. Open this file with Notepad++ or similar application. Go to the bottom of the file to the Display setting, change the value for brightness from 80 to 100, that gets rid of the dark filter. Now scroll up a little and find the "VCM_LAYOUT_9_EXPLANATION" label, change the TV size from 928 696 to 1280 720, and the position to 0 0, then change the part below labeled as "drc" for the target, so that the size is 854 480 and the position is 0 0. Then save the .json file and transfer it back!
You can edit the layout and sizes of the other options, but my favorite layout is the Top screen on the TV and the bottom on the Gamepad, so that's all I've focused on changing here.