So your claim is that Serini might have faked being two different people, while being invisible, after having already knocking out the two Paladins so they won't even know about it, and with there being no else around?
That feels less "improbable" and more just "nonsensical". And nothing about Serini's current mindset or her actions strikes me as nonsensical.
But the very comment that started this thread wasn't directed at casual readers. It seems very much calling out people who obsess over every little detail. But even then, you don't have to obsess over every little detail to think "the last remaining member of this important group who hasn't been confirmed dead is likely to show up when they get to her gate." I feel like that's just the kind of thought you have if you're basically just up to speed with things.