If Marinette is unavaliable (Maybe Off to Italy or London....) when Stoneheart appears... who would be the Ladybug and Black Cat Holders?
Safest bet is Adrien and Alya.
Marinette being gone wouldn't effect Fu picking Adrien at all; honestly the most up in the air is who would be Marinette's partner if Adrien's not there. Especially as Fu picked someone after school started so who else could be out there? Or do you go a scenario of Fu doing another test before Marinette's which leads to more options.
Anyway!
Alya actually comes like, a minute behind Marinette in Origins, and mindful, we don't know which way she' s coming to school, but for the sake of this, she could've been not too far from where Marinette was, and did that test in Marinette's place.
And I'll just go ahead and assume that Alya gets Ladybug and Adrien still gets Cat. Based on what's seen of canon, Alya would be the better pick for Ladybug between the two.
And for how things go...
Well, like canon, Alya will be more focused on figuring out her name, Adrien is more goofing around and enjoying being Chat. Their partnership is far friendlier than canon and they both like to rush in so probably insta click as friends.
And that first fight between Stoneheart, Alya would listen to Tikki excitedly so she's more likely to remember what needs to be done. At most, her trying to figure out her hero name could distract her but I ultimately see her getting focused near Stoneheart and it probably goes better in canon.
And this being Alya excited about being a hero and needing to tell someone, as Marinette's not there and she doesn't move from what's Chloe's seat in canon, she tells who's behind her: Alix and Mylene, aka, your future Bee and Turtle (not sure who would be what yet).
And so you have a friend trio. Alix eggs Alya on for chaos (while occasionally being level headed), Mylene is trying to be the voice of reason and fight off panic attacks, and Alya is here to be that leader who is also the hero between them.
Adrien may or may not be aware that people know about Scarabella? If so, he may out himself to Nino so Nino's in the know. Probably future Fox.
And partnership wise, they actually may not have much issues thinking it over. Biggest reason Adrien was difficult with Alya was that he had a crush and bias for LB and make jabs at Alya for not doing as well as her. But there's no LB here, so nothing to compare Scarabella too. And when she gets bossy, he may not mind as he's cool with most people bossing him around (like Chloe). They may not be as efficient a hero duo, but they may be similar enough to get along well. Though I could also see Alya loosing patience with him goofing around here and there.
I also don't know if Adrien and Alya would even crush on each other and vice versa.
Either way, with Marinette not there, Paris will be in decent hands. Alya can be pretty sharp and effective, she just needs time to think a little more. And maybe this will force Adrien to grow and step up as a hero and be more involved in the planning as Alya's not going to have everything down plan wise like Marinette would.
