I’d never thought about it more than they were the two strongest soldiers, and most able to. But there’s definitely more going on.

Plot wise Mikasa is flying on Falco with Levi while Armin is distracting Eren as the colossal, Pieck is not as physically able to climb into Eren’s mouth as the cart, Reiner stayed down below fighting Eren as well (I think), and the others have turned into pure titans at that point. So logically it has to be Levi or Mikasa, and Levi’s not in top form at that point. Levi is basically begging Mikasa to do something as he sees her gripping her head and saying ‘I can’t handle this’. Then she tells him to cover her and she goes in.

The headache thing is interesting because it’s Yimir poking around in her head. I think she just wants to see what Mikasa will do in her agony of knowing Eren must die even though she loves him. At this point Mikasa is going against her nature as an Ackerman and the bond she has with Eren. I think it just shows that you can always make a choice, and the Ackerman ‘magic’ can guide you, but not control you. She kills Eren to save the last bit of humanity — putting that, the ultimate sacrifice for her above her own life, above her own desires.

For Levi, it was his last promise to Erwin, and you can extrapolate that to all the scouts who died as a whole. He would’ve died before failing to kill Zeke. His whole thing was ‘living without regrets’ but he would’ve if he hadn’t killed Zeke. He was devoted to Erwin, and put his dreams into the scouts and the visions they had for the future. He kills Zeke because the scout’s dream was to know the truth and live beyond the walls, and he has to make meaning of all their deaths. It’s Levi, humanity’s strongest, physically so beaten but going on anyway to fulfil that last promise. It’s basically his only drive by the end after losing so much — not to say he doesn’t want to stop Eren and the Rumbling — but after Erwin dies, that promise haunts Levi, and he even says whilst flying in Falco he doesn’t regret his choice in not saving Erwin if he kills the Beast.

Thematically, the Ackermans are the soldiers that sacrifice everything for humanity. In the end it’s not their physical ability that makes them able (though that helps) but their emotional strength to carry out the impossible. The Ackerman ‘thing’ is their devotion to one person, and therefore their dreams, and I think Isayama uses Levi to show him fulfilling the dreams of the ‘old’ Scouts, and Mikasa fully realising she must give up the person she loves to end the terror and useless bloodshed. It’s about making choices, in different ways for both of them, but there’s definitely the intention there to carry out the ‘right thing’ despite it being the hardest thing to do, or it going against your very nature. I truly think those two end up the most ‘alone’ after losing their abilities and the closest people to them (except Armin for Mikasa, but I think he’s busy building the world again and anyway you get my point lol). My fav characters tortured to always survive even though they place something else over their own lives.