But free Botox without all the scary needles?

Good post, op. I had no clue and had been thawing fish in the plastic for forever.

Oxygen, I think is the major factor. As long as the salmon is frozen and sealed, the bacterial growth is slow/spores remain inactive. When thawed while sealed, they are all trapped in there without oxygen and become active with the increased temperatures. According to some quick wikipedia search, it is the germination of spores into bacteria that creates the neurotoxin. The absence of oxygen is a requirement for the germination, so it promotes neurotoxin production. Releasing the salmon from the package before thawing lets oxygen in, thus they won't become active as the salmon thaws, since oxygen slows down the bacterial growth.

Not an expert so take it with due caution, but as I recall when canning things like meat and I think tomatoes you have to use a pressure cooker rather than just boiling water to get the temps past 100 C, so I guess just cook the shit out of it?

The botulism bacteria is anaerobic. It is not active the same way in the presence of oxygen. So when you open the bag and remove the fish, you are introducing oxygen.

When fish do it, it's called spawning

The fish won't mind.

I have lived my whole life without seeing single wrappers or worrying about "freezer burn". Do you mean feeling cold when you touch something that's cold? Even when pouring a liquor that's been in the freezer it hurts in the moment but it's nowhere near an actual burn