I just think that it's reasonable that a tiefling wouldn't know their subrace but I wouldn't protest the presence of options that assume they do, since I'm sure some tieflings could figure that out. I just doubt that they would know as a rule or that they could find out without dedicated research.
As for Zariel herself appearing, I'm all but certain that that is a consequence of them having directly worked for her. The power average tieflings have is a result of their heritage, same as some folks having brown eyes or curly hair. It's not something she directly gives to them. Karlach was different in that she was directly empowered by Zariel. I also think that the name carving thing wasn't a dream, it was our character seeing a literal memory of a thing that actually happened to Karlach.
