I’ll give you that Lynette is a pretty good measure for how most people think about the second queen. That everyone (and even Fie) believes the marraige is legit. However there should have been some paperwork or ceremony that a Fie would have to be a part of. Which we never see or is mentioned. Plus Roy doesn’t want a wife, let alone a second one under suspicion of hurting another member of royalty.
Why saddle himself to someone he thinks is a potential criminal? All the book says for sure is that she is a part of Fielle’s dowry. If Roy was only thinking politically (even though it’s still a stupid decision) why not accept the girl, but not marry her till after the investigation is complete. After all rumors have already assumed Fie is the worst. Why not have a rumor that there’s a second queen, without the actual paperwork? It’s not like anyone of importance talks to Princess Fie anyway.
Maybe all that Fie really needs is a fantastic lawyer.