The spaceman thing you're talking about is a sarcophagus lid from Palenque. But rather than fraud, don't you think it's just having a good imagination, and tapping into the zeitgeist of the time, all about ufos and moonshots? (plus having an eye on what makes a bestseller). I mean I speak as someone who devoured the books as a child.
But don't drag mr fort into this, he just reported things. And he was dead by the 1930s so apart from weird victorian zeppelin scares, he didn't really do space rockets :)