I would take pictures of what you have right now. Then, whenever this company comes out to do leak detection, make sure you get their business information. Tell them exactly what has happened and ask them to do leak detection on the skimmer, returns, light fixture and main drain. You can certainly tell them to focus on the light area and to show you what they are doing.
I know it's really annoying to have to do this but you need to sit on these guys and watch everything they do and take pictures as well. My gut feeling is that something go screwed up at the light fixture with the liner installation as the leak stops there and you have said that the light fixture bulges out on one side or something like that. Take pictures of that area specifically (you can post some here if you want). You have to understand that whoever did the liner install is potentially on the hook for a brand new liner (thousands of dollars, I presume) and if they screwed it up it's going to come out of their pockets which means this one job will have cost them tons of money (all loss no profit). Reputable companies would have business insurance to cover against these types of losses; fly-by-night operations and small shop companies often skimp on insurance or don't carry any at all and take a gamble. The last thing they want is to have a screwed up liner pinned to them. So you have to be vigilant and get all the facts so that if it is their fault, you can confidently demand that they make it right.