I exported the 1 piece light, changed both the obj. and mtl. files, though you only said to do the obj. file, but previously you said that they both have to be changed whenever I change something?
Again, knowledge you would have gained when you had started of a slower instead of diving directly into advanced topics 
There's another manual on dodecagon.nl that explains changing material names. Reading that manual will make it more clear what happens when you change material names and the relation between "usemtl" in the obj file and "newmtl" in the mtl file.
Make sure you have the level elevations as mentioned in my previous post.
The Brace
Export the walls that are the Brace or export the brace you already have if it is correct.
Check the obj and mtl files and change the material name to "Brace" if you hadn't done that yet.
Import the Brace obj file and check that it now only has one material name: "Brace".
If you exported the walls again you will now probably have an object that is 6" higher. Change the height to the desired height. (The walls are higher because of the added floor thickness.)
Not many users know this but since the brace only has one material you could simply remove all lines starting with "g " and all lines starting with "usemtl " except the first one of each at the top. Actually, only the "usemtl " line is necessary in this case.
The panel
Export the room and change the material name in the obj and mtl file to "light".
Import it and check that it's height is correct and adjust it if not. If the height is 0 you have not set the level elevations correctly.
Combine
Insert the panel in the brace. Export both together. Import and check that you have an object consisting of both the brace and the panel with two material names: "Brace" and "light". Change the colors to what you want, turn/tilt it to the correct position, and export. This is the final correct Brace that you import into the library. You can import it to check that everything it correct.
Import the final object into the library and set the properties as mentioned in my previous post, especially the lightSourceMaterialName.
Some of your remarks let me think that at some point you used an older version instead of your latest export. Sort your filemanager by date so the ones on top are your latest export. Makes it easier to avoid using old versions. Even better, if you are sure which is which, only keep the latest brace, panel, and combination and move the rest to a subdir 'old'.
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