As a small nonprofit with limited funds, this sounds like a true nightmare.
- hundreds of dollars a day in sudden, unpreventable costs
- no support from the company (who benefits directly from your suffering)
- code installed on IoT devices that can’t be easily fixed
freeCodeCamp has been in some bad situations before but nothing like this.
I would go forward with the assumption that Firebase will continue to ignore you (though hopefully the success of this post will spur them to action).
You mentioned going door to door upgrading the firmware on these devices. How big is your install base? Is that plausible? How many paying customers do you have?
Could you make some sort of kit you could send out to customers that could enable them to update it themselves? With IoT I imagine that these sorts of updates are necessary anyway sooner or later.