With FSX, you dont really need to name the textures with a _L on the end, and alot of times, I have light maps that dont have that as well on FS9 models.
But, on your portovers, its difficult to say. First, dont run them in DX10, of course. (I know you knew that, but for others to know). You 'could' tweak the lightmaps a tad. If they are not bright enough, for instance, you could open them up with a graphics editor and brighten them up. Useually you only need to copy/paste them from one folder to all the other texture folders, as lightmaps are usually all grays. (Some can be colored differently though, like on the Epic panels that have different wood and metal panel types). But grays, can all share the same texture in their various folders.
Another thing is that Wozza discovered that Portovers will run DDS textures perfectly. Try converting and flipping them all to DDS. Its alot to do, but if you like the plane(s) that much, then its worth it. But...... I dont know if that will cause a improvement in the night textures issue. (Usually only frame rates improvement on that route).
Good luck on that. Hope you get it all working properly.
Bill