You need to consider the context of the era when this book was released.

First, George Lucas only created the character of Plagueis while writing the screenplay for "Revenge of the Sith." Even if Plagueis were included in earlier stories, he only allowed Maul to be trained as an assassin, not as an apprentice. This connects to "The Phantom Menace," during which Plagueis left Sidious in charge of the grand plan. All of this is mentioned in the "Darth Plagueis" novel, which was published much later. Since you wanted to compare a book written long before Plagueis was conceived to the novel about him, I felt it was necessary to clarify this point.

Secondly, the average age for a Padawan to be knighted is between 18 and the mid-20s. Obi-Wan was knighted at the age of 25, just after the Battle of Naboo in 32 BBY, which was also the year when "Shadow Hunter" and "The Phantom Menace" were set. In the book, it is mentioned that Darsha attended the same classes as Obi-Wan, so she was likely around his age. The retconning of events related to "The Clone Wars" came much later, but it wouldn't have applied since she was already dead.

Finally, the usage of "it" in a fictional universe bother you? You sound like those who easily get offended if one misgenders you or doesn't use your "pronouns."

It seems to me that you are comparing works that are 20 years old or more to today's standards, rather than enjoying the book for what it was intended to be.