Lead toxicity wasn't some big unforeseeable surprise, there was over two millennia of documentation on the dangers of lead when Midgley kicked off the mass production of tetraethyl lead.
It's better to just say the man was cursed. He also brought Freon to market. Arguably no living person has done as much environmental damage as he accomplished single-handedly. As Bill Bryson said, he had "an instinct for the regrettable that was almost uncanny."
Two years after he got lead poisoning, he did a demonstration where he huffed TEL and poured it on his hands and claimed he could do it every day without consequence. Nobody believed him, perhaps because it was no secret that lead exposure was dangerous. The state of New Jersey didn't believe it either and shut down his plant, and he later came down with lead poisoning again and had to take a leave of absence.
So to your point, these were the actions of a man who saw people in his factories dying, came down with lead poisoning twice himself, and kept selling the stuff. That's not a lack of foresight, that's just greed. He just didn't care. The benefits he saw were "making a shitload of money".
I can't imagine he knew about the dangers in that case. Even someone amazingly greedy would be unlikely to risk the very serious complications doing this would cause.
If you yourself have to take a leave of absence (admitting the cause) and dozens of your employees become sick with the same thing (many of whom die), it's certainly not unknown. Whether it's unappreciated or ignored is different. In this case I don't think we can know for sure.
I would speculate, though, that Midgley knew exactly what he was doing. After the state of NJ shut down his plant, the feds had it reopened. TEL was important for military purposes. Midgley knew that he found himself possessing a key ingredient for internal combustion. It's not often that you can insert yourself as a key component of an existing (fifteen year old!) industry. He knew he could put on a show and downplay the risks, and if he pulled it off he'd be rich like a Rockefeller.
reminds me of the guy who made millions (billions?) selling identity theft protection theatre (LifeLock IIRC) by running an absurd ad campaign in which he put his own name and SSN in public advertisements for the service, and promptly had his identity stolen, and kept having stolen over and over for years.
But with hundreds of millions in the bank selling this scam he likely had plenty of other resources to mitigate the damage, resources his millions of customers likely do not.
Morale: Don't follow the man with the seriously damaged IQ.