The shop owner was an ass, based on your description, but didn't lie. How you build for an RTA and how you put in the cotton will cause the issues you had. All RTAs are going to leak when the wicking isn't correct, and any atomizer with a clear path to the coil can shoot hot vapor into your mouth if your build is too hot for the atomizer when it produces the amount of vapor you're looking for.
As dshobe said, the shop owner is no longer allowed to correct it for you or teach you how to correct it, because of new the FDA regulations. Watching tutorials on Youtube is probably the best way to get started with RTAs. I watched at least 2-3 before building on an RDA, and did about 5 RDA builds at least before attempting to build on a RTA. My first RTA build leaked at first, but the wicks expanded or ?????? and it settled down and was a great vape until I had to rewick.
The thing about rebuildables is the customization, but that's also the challenge. After only a couple of months of using them, I have a much better idea of what I need to do to wick an RTA (Coil size determines wick thickness which needs to match wick hole size, etc), I can tailor a build for a type of RTA, or for a mod (RDTAs work very differently compared to RTAs, and a DNA75 loves a build over .5 ohm to run lower watts for longer battery and less low-bat errors) There are so many factors! They are definitely a 'do the work, get the reward' endeavor. Stick with it, it's worth it.