When it comes to safety issues, I would suggest havng a plan in place. Could you, for exemple, dedicate a cabinet for all your chemicals, go to the hardware store and install a lock with combination. This way, you won't have to worry about hiding a key. If you're not good remembering numbers, they make some locks with letter combination. I'm sure you already have done so, but try to "child" proof your house as much as possible. Sometimes, I feel my house is kind of empty... but at least I don't have the stress of having everyting destroyed.
Try to make a list of priorities Some things can be put on the back burner for now. Some others, it will be worth the battle. Safety being the top priority and bad eating habit will be tackeled later.
Don't try to be the "perfect" parent: take one thing at a time and it will relief some stress.
I have rarely used it, but I also have his bedroom with the lock on the outside. I think I've only used it 2 or 3 times in the last 2 years. Maybe it's something you might want to consider. If you really need to get some done and he cannot be trusted. At least you know where he is and you don't have to make it sound like a punishment. Maybe play with him a little and explain that he will have to stay in his room until you're done with whatever you need to do. You might get lucky and he migh actually listen. Or am I delusional?
Just a few tips.