Say I have some creature out, Glissa, the Traitor, Shimmer Myr, and Grave Pact with out Executioner's Capsule in the yard, while my three opponents all have a few creatures out. If an opponent kills my first, arbitrary creature: each opponent sacs a creature to Pact and I get 3 Glissa triggers. Could I get back the capsule, flash it out, use it, then use her 2 other triggers to repeat this process?
You cannot. Because Glissa's ability targets you must announce a target when it occurs. Whenever the Executioner's Capsule changes zones it becomes a new object so when you get back the capsule, flash it in, and use it once it is in the GY it is a new object and the original Glissa triggers will be countered if they were targetting the original Executioner's Capsule.
A better option would be to use the capsule on your creature in response to their removal spell (or on Shimmer Myr, if you must), to put it in the graveyard and make it a valid target for one of the Glissa triggers (and the creature that just died another valid target, as long as it's an artifact creature). Then flash your creature and the capsule back out, and you can repeat the process, with both the artifact creature and capsule being put back in your GY each time to be picked up by the new set of Glissa triggers generated by your creature dying, so long as you have mana to keep recasting the creatures and activating the capsule.
The Grave Pact is a non-essential piece of "this process"; it just gives you a head start in killing some more creatures to begin with. As long as you have only one Executioner's Capsule, all you can do is make it kill one creature for 2B, trigger Glissa, and target a Capsule card with her. Since the Capsule you used is in the graveyard, you can recur it.
If you Execute your own creature, you get a Grave Pact trigger, and off of that you can get back up to three Executioner's Capsule cards, if you have three of them. Targets are chosen as spells or abilities are put onto the Stack, remember.
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If you Execute your own creature, you get a Grave Pact trigger, and off of that you can get back up to three Executioner's Capsule cards, if you have three of them. Targets are chosen as spells or abilities are put onto the Stack, remember.
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