Riven, Broken Blade (Alternate Art)

Riven's buff costs appear later in her triggered effect, so you spend them during resolution, not during finalization.

The "Spend my buff to..." wording makes spending the buff a cost within instructions for the linked [Stun] or ready effect.^740.4.a Riven's effect starts with "choose one," so those costs appear later in the triggered effect. Costs that appear later in a triggered effect are paid during resolution.^204.3.a

Choose the mode and any target it requires during finalization.^355.10403.2 If Riven has no buff when resolution reaches the chosen instruction, you cannot spend one.^702.2.b.1 Because the cost cannot be paid, the linked [Stun] or ready effect does not occur.^203.3

Yes, as long as that mode has not already been chosen this turn. A unit that already has a buff can still be chosen for an effect that would buff it.^427.1.c You choose Riven's mode during finalization.^403.2 When the ability resolves, Riven receives no additional buff, so the chosen mode has no effect.^427.1.b.1

Yes, if Riven is ready when either replacement effect applies; no, if she is already exhausted. Both Zhonya's Hourglass and Guardian Angel replace Riven's death with instructions that heal, exhaust, and recall her.^369.1370.1.b

If Riven is ready, exhausting her changes her state to exhausted and fulfills her trigger condition.^{415.1370.1.a383.2.c} If she is already exhausted, she cannot be exhausted again and the instruction does nothing additional, so her condition is not fulfilled and her ability does not trigger.^{415.1.b415.1.c383.3}