Could a measurable amount of isopropyl alcohol occur naturally in an otherwise reasonably earth-like atmosphere? And if so, what is the highest possible concentration that a human could survive long-term?
Yes, assuming that pools of isopropyl alcohol exists
The atmospheric temperature, in general, is too low for isopropyl to naturally exist in it's liquid form. (That is, isopropyl alcohol has a boiling point of roughly 180 Farenheit, below water.) The reason why there's water vapor in the atmosphere is a property of liquid - every liquid has a small amount of gas forming on the top, subject to various factors such as heat and the other gases around it. And since the ocean is massive, the amount of water vapor formed by evaporation is also massive, and thus exists in the atmosphere.
You can use Henry's Law to figure out how much isopropyl alcohol is needed, I think, but given that I'm not an atmospheric chemist, the exact equations needed are beyond me. I just know it's possible given massive amounts of isopropyl pools. How you get these pools must be addressed in another question, because isopropyl alcohol does not occur naturally, and is in fact poisonous (like most alcohols) to organic life.
Inhaling isopropyl alcohol is dangerous. In high concentration, it causes the following symptoms: Coughing. Dry/sore throat. Central nervous system depression. Dizziness. Headache. Narcosis. The recommended treatment is fresh air exposure. But that's high concentration only, and given that water vapor in atmosphere is at a maximum of 4.0%, and the fact that your isopropyl alcohol pools will not even have the volume of 1% of the world's oceans, you will never have to worry about naturally occurring isopropyl alcohol reaching the level of lethality to humans. Unless they do something dumb like going right next a pool and taking a few deeps whiffs.