Yes, it is very possible that "engineered" fire-resistant wood (and all the added chemicals that come into play with that) could be toxic, one way or the other,

Yes, it is very possible that "engineered" fire-resistant wood (and all the added chemicals that come into play with that) could be toxic, one way or the other, we are already using in construction lots of "engineered" (composite) wood, and I am personally not much convinced that even that is really-really "safe". As a reference, for formaldehyde emissions: https://en.wikipedia.org/wiki/Formaldehyde https://en.wikipedia.org/wiki/Formaldehyde https://en.wikipedia.org/wiki/Formaldehyde#Safety https://en.wikipedia.org/wiki/Formaldehyde#Safety it took decades to categorize them as toxic/carcinogen and then put limitations in Laws and building codes.