I would like to hear others take on that as well. Honestly never thought too much about on it 
Generally I would suppose the exhaustion got to her as she knew the heir was on their way and just wanted to be done with waiting. Kinda on a… “Either I take a chance and trust this noble, or I don’t and let the attacks and public unrest rise to a boiling point.”
Although @EmperorHeartless How would you prove to her that you are who you say you are? Material objects could be taken off a corpse…
To me there could be several ways. On the top of my head…
For one there is a distinct magical presence in the world. Maybe an Avor can be specially tailored to people and could only be activated by the legitimate owner unless passed on by the owner’s expressed consent.
While you are right about material objects that could be stolen you could still have a signet ring that represents your house and would only be given to heirs of the Noble house. While it could be stolen it is at least an identifier and let’s face it if a Noble can’t keep track of one thing… that doesn’t bode well for his/her organizational skills.
The way I go about things if there is a difficulty in explaining something and I really don’t want to change how the scenario is going I take the time to write in some extra lore that could help as an identifier for the MC to help explain certain events in a legitimate way (like an Avor being tailored to show that the user is who they say they are)
Those are a few things I can suggest atm.
Also even if it’s in desperation just trusting a supposed Noble and putting them on the throne could lead to complications in the future. For instance say that supposed Noble is a fake, becomes the ruler and then the actual Heir shows up and is basically WTF.
It gets even more complicated if the Fake Ruler is actually a decent or better leader/ruler than the legitimate Noble ruler.
Of course you could also have Serenity approach your PC later and speak in private where she voices her reasons for so easily taking your words at face value like…
Serenity: “I’m still not convinced you are the true lord of this House”
PC: “If you aren’t then why prop me up on the throne?”
Serenity: “Simple really. These people need a leader now, before someone comes and bulldozes them all done and pillages our city. I am taking a big risk in accepting you so soon, but listen well. If I find you unsuited to rule or a fake I will be the first to demand your head on a platter”
or something to that effect.
Looks like we’re going with the signet ring idea and a will.
News: Word on the street is that the reevaluate update is around the corner. This week to be exact, thanks to alpha testing. 
cough cough
What’s with all the winking.
I can’t wait to pile up peasant bodies! 
There will be always more peasants to pay taxes no matter how many i kill.
When oh when will this nation of ours get a noble ruler that actually love their people? I can just hear the cries of the people demanding a just rule.
@Emptiness Then again… is there a need to harm your subjects?
Because is amusing? +20
Welp, at least we know who the Inquisitors will be visiting…
Who is currently on the throne?
It’s… complicated.
No one technically. As each house feuds for domination over all of the land, there is only chaos.
However! There is one person which every province recognizes as an authoritative figure who acts as an independent… observer. They are the one who will decide if a noble is worthy of the throne. They are the one who reigns over the inqusiitors and punish the rulers who would further the realm down the path of destruction. The Lady Divine, The Supreme of The Divinity.
And when i take the throne i can unite the realm into an empire? With me as the absolute power of course.
To take the throne, you would have to unite the realm into an empire. For anyone can simply lay claim to the throne and rule] of Imortallia. It means nothing when opposition against your rule is met everywhere. Thus you must first defeat anyone that would stand in your way to domination.
How you do it, is up to you.
What about our paragon Sebastian 
SORTA 
what does that mean…the things you say to your poor readers
