its not too far-fetched but the problem and the reason they're trying to get in Shinganisha (probably spelt wrong) is to get to the basement but at the moment, they are being held back by the beast looking titan and other titans under his command. and now they have many dead and injuried soldiers even with the retreat of the beast like titan guy. Bertolt is with Eren and crew while Reiner was taken by beast like guy. Eren could seal up the two gates that were destroyed by them but they now have time on their hands due to both Armin and Erwin being in very bad shape. and not only that, they'd have to kill any remaining titans before they can even think of getting to the basement.
yes, I think they will both be saved. There are so many ways the situation could go I can't even begin to list them all. I think I know how it will happen. I don't think it will be the basement. I sort of think the basement might be a red herring. I think it will happen through the actions of the characters. I believe that Levi will play a strong role.
i don't think they'll both be saved because the serum can't be shared and i think isayama has been stalling erwin's death for too long to not kill him and let him live again.
kill him and let him live again? make up your mind, will ya?
b. splitting the serum is the only option? No. It isn't. If it was, Isayama's story would be in big trouble.
you misspelled spelled (not spelt). That's such a cute mistake.
don't forget that the serum is only half of the equation. the injectee also needs to eat a titan shifter. Otherwise the injectee turns into one of those idiot titans Levi's always killing.
What if there is more serum in the basement? Dr. Yeager injected Eren with the serum when he was a kid, so the question is, where did he get it? Maybe he knew how to concoct it and there is more in the basement. Maybe Erwin and Armin can both be saved. I don't know if this sounds too far-fetched. It's just a thought that crossed my mind.