What happens to Mary Sioux at the end of the movie?

Mary Sioux is grievously wounded during her brutal rematch against Tanya at Tiegenhoff. She is rescued from certain death by Lieutenant Colonel William Drake, who orders a retreat for the surviving members of the 42nd Flying Division.

Does Tanya get to retire to the rear lines as she wanted?

No, although Tanya successfully convinces Strategic Headquarters to let her spend two months researching combined arms tactics in the rear, her plan backfires. Zettour assigns her to command a brand-new combined arms unit, putting her right back into active service.

Why does Tanya suspect Being X is involved with Mary Sioux?

During their fierce aerial duel, Tanya notices that Mary exhibits an abnormally massive and unnatural amount of magical power. This reminds Tanya of Being X's interference, leading her to believe the entity is using Mary as a tool against her.

Why is Tiegenhoff so important to the Empire's military strategy?

Tiegenhoff serves as a major railway hub leading directly into the Russy Federation. Capturing and defending this location grants the Empire critical control over supply and transportation lines, effectively halting the enemy's advance.

Who leads the 42nd Flying Division during the battle?

Lieutenant Colonel William Drake leads the multinational 42nd Flying Division volunteers, which includes recruits like Mary Sioux, as they train and eventually enter combat alongside the Russy Federation forces against the Empire.