Can I play Shen as a reaction during showdown, then play Call to Glory to buff Shen (by removing a buff counter from Sona) to kill the attacking Jinx and save Sona?

Ruling: No, this sequence will not save Sona. When you play Call to Glory and remove the buff counter from Sona, her Might will drop to 6, matching her 6 marked damage, causing her to die during cleanup before combat damage is dealt.

Sequence:

• Jinx player attacks
• You react with Shen (Shen resolves immediately as a unit)
• Focus passes to Jinx player, who passes
• Focus returns to you, you play Call to Glory paying 3 by removing buff from Sona
• Sona dies during cleanup (Might reduced to 6, matching 6 marked damage)
• Combat resolves: Jinx dies, Shen survives with Tank

Nuances:

• Units and gear resolve immediately and do not linger on the chain
• When units/gear resolve and the chain closes, focus passes to the next player (unlike add abilities which have a special exception)
• Showdowns only end when both players pass focus consecutively without starting a chain
• Removing a buff counter that is keeping a unit alive will cause that unit to die during the next cleanup