I guess you're playing devil's advocate here? But to answer your question if the steps here are followed carefully then it is unlikely that anything will go wrong. Nicotine ditartrate, benzoate and others have been added to cigarettes by BT so I guess the hazards are lower than the combustive chemicals ingested from cigarettes...tar, carbon monoxide, benzene.
Just to note benzoic acid has absolutely no relationship with benzene (a well know carcinogen and all round nasty substance). The only similar part is maybe the ''benz''. Worth noting here as I am sure that is what some of us do
Further more benzoic acid is used as a food preservative and its toxicity is very low to humans so no reason to get unduly worried about vaping it, especially in the very low quantities that it is found in a nic salt eliquid