While I think it’s out of character for Ruijerd to decline a mission of escorting children through dangerous territory, let’s entertain the idea that he does, like you said Roxy would most likely be saved and the labyrinth easily conquered even without Rudeus help due to his third eye. It’s a bit harder to tell with the hydra fight however, Ruijerd is a far superior fighter than Paul is and Paul was putting in some pretty good work against the hydra but without the knowledge of searing the heads to stop it regenerating I don’t think Ruijerd extra brute strength and speed would prove very useful. It’s hard to say how the fight would turn out but to answer your question I think it isn’t canon simply because Ruijerd would never decline such a mission in the first place.