The issue is Arcadia doesn't really want more power or wants to be the Queen. She already has a high standing. And like you said yourself in your other post, she's devoted to her family, the kingdom and her studies, I suppose. She also wants stability and fairness, from what I recall. She has no reason, yet, to entertain Elis lust. Especially since it's unknown if her husband died in the east or not. Just that he's been gone for over a year now. She doesn't come off as the type to do such things just because a King or Emperor offered it.
Unlike with Valeria, who is off in the west and was already planning to kill her husband with drugs. Even implying that her husband isn't even the father of Valerian. So for her, she's getting a future king lover and his bastard. Valeria already thought ahead about some of this stuff. She's different from Arcadia in that sense. Since she was doing this long before Elis was born basically. She didn't love her husband like Arcadia loved hers. She was already sleeping around and continued to do it after they were married.