BIG HUGS
I am so very sorry for your losses.
Summer and high temperatures/high humidity cannot just cause an increase of human deaths in the elderlies but also the same in frailer/older guinea pigs. I've lost quite a number of my own oldies during or in the wake of sudden heat spikes or heatwaves even if I managed to keep temperatures within bearable limits in my piggy room and they were not plastered to any cooling elements.
Heart attacks and strokes are sadly not at all uncommon in this context, unless it is the overloaded immune system that is giving way to whatever bug can breach it.
Perhaps it helps you to put the stroke event into that added perspective and be a bit more at peace with it? When the chips are down despite their huge personalities, guinea pigs are just small animals with a much faster metabolism that can turn against them.
A sudden death/fatal event is generally very traumatic because you cannot brace for it. You have the shock to deal with as well as the pain the of the loss. However, you have done absolutely the right thing to spare your gorgeous Zira any further suffering and more strokes in quick succession; you have not failed her in any way. Your heart is in absolutely the right place. What has happened was simply not in your control and could not be prevented.
Please feel sad but try not to feel bad. You are clearly a very caring owner who has done their best. It has helped me as well on occasion to picture piggies that have died in very quick succession together at the Bridge and to mourn them together as a pair. They will always live as such in your heart and in your precious memories, even if it is too painful for you to go there right now.
Take good care of yourself and be kind with yourself in the coming days. It is OK not to be OK for a little while but please seek help if you struggle to sleep or function, or if you get trapped in one of the weird mind loops that can come with the onset of the grieving process since we humans are wired to reflect everything back onto ourselves and our soul-searching can take us to some strange places indeed.
Unfortunately, a new death can bring up previous ones that are still being processed themselves and add to the emotional load - especially when circumstances have been somewhat traumatic for you. That is not at all nice but normal, and something that only time can heal.
Here is the link to our Grieving guide for human owners that talks you through the grieving process but also contains plenty of practical tips of what you can do for yourself, so there is hopefully something for everybody that vibes with them:
Human Bereavement: Grieving, Processing and Support Links for Guinea Pig Owners and Their Children
Try to take care of yourself.
Is Zira leaving a new companion?