Ok, if you have the space to spare, why wait for the failure to go fetch the emergency ice? Just fill the empty space with ballast water and you're ahead of the problem.
Once the thermal mass is at the desired temperature, it will help bring newly introduced items to the desired temperature faster, not slower.
The only time it slows down the cooling is when first introduced @ a warmer temp, or when there's been a failure for long enough to let everything warm up. But note that in the failure scenarios, the thermal mass helps keep the space cool for longer than had it not been there. Just remove the warmed ballast when the failure is over to let the perishables re-cool first, then reintroduce the ballast when appropriate ... perhaps even pre-cooling it elsewhere first.
You want something that has a thaw point way below your foods thaw point - add salt t your ice.
But you're really better off adding salt to those bottles so they absorb energy when melting below the freezing point of pure water (and frozen food).
If you want to keep tap water, you can buy silver ion additives at any camping/outdoor store. If you’re really planning for preparedness, buy a ceramic water micro filter and additives. That can make nearly all water usable.
It is a public health concern at scale.
Two overlapping but divergent scenarios.
(Or gazpacho, thought that might be shot down as a bit niche though. But it is lovely this time of year (in the Northern hemisphere)!)
I've known people keep celebratory champagne permanently chilled in case of an unexpected call for it; I semi-frequently put beer or white wine in the freezer for 15-30mins ish because I didn't think to put it in the fridge earlier (or had to take them out to make room something more critically needing to be kept cool).
Nothing to do with food safety though.