I'm not a bike shop employee at the present, but I worked in a shop for nearly 10 years as a mechanic. So maybe you think I'm biased. Looking for cracks is not a thing. Stressing a frame to look for cracks is not something that is ever done. I know my lbs has found cracks in frames, but I'm sure they have had frames with closed cracks like the one you describe that they haven't noticed. I don't think I have ever seen a crack like the one you described and wouldn't think to check for it. If someone brings in a bike without reflectors and doesn't ask for a reflector, then it wouldn't occur to me that they wanted a reflector. This does happen, people bring in bikes and ask for reflectors to be put on. More people come in and ask to have them removed. Is you kid going to be riding this after dark? Most states have a law that requires lights and reflectors 30 minutes after dusk.
Running a bike shop is anything but lucrative. The owner of my lbs told me how much he makes one time and I'm sure he would be making more elsewhere. Without all the headaches and difficult customers. Mechanics are poorly paid. And my lbs is one of the few that pays for health insurance for the mechanics. They need to make sales to keep the lights on. It occurs to me that you are never going to find a bike shop that will make you happy. I know there are a lot of mediocre bike shops out there, but none of the things you describe make me think that there is a problem with any of those shops. Well, maybe the guy that didn't like kids. I would save time and start watching videos on the park tool website and buying tools.