Sounds like the cleanup costs were largely related to the fact that the reaction caused an airtight drum to explode and spew radioactive waste throughout the facility, though, which presumably wouldn't apply to the "metal fire on an aircraft" scenario.

I'm curious what would actually happen, worst case.

Assuming the metal fire couldn't be extinguished, could it at least be contained to melt a small enough hole in the aircraft to safely land?

We had an incident where a laptop with a swollen battery fell and lit up in a public way. It attracted attention and some research was done - they realized it happened a dozen times a year. Hazardous disposal options vary by location. So the question became… what should be done with these compromised batteries before they get disposed of?

It’s a simple thing that costs nothing. It’s like a fire extinguisher to me - I’ve never experienced a fire at work, yet we have extinguishers and exit signage everywhere.

That said, the rate of burning batteries is very low. (Like 0.001%) Unless you have a ton of people and different use profiles, you’ll never see this happen.