Regarding smoking, yes it's possible, but less likely than other methods of ingestion due to inability to control the amount actually delivered. Just due to speed of incapacitation and a limit to how much can be vaporized and inhaled at once, F would be far more likely than H in this case.
Any full mor agonist can cause death provided equivalent dosages are used. Oral fentanyl has a bioavailability of around 30%, and, in a non user, heroin is 15-20x more potent IV than orally.
the biggest issue with H is that you have no clue what purity you will find on the street, so giving numbers that are likely fatal is irrelevant. With F, it can be found relatively pure. Intranasal usage may be a better option than orally if one doesn't wish to utilize IV.
all that being said, if you have no experience with IV opiates, you might wish to look at the resource section aNd other methods.