I like number 3's explanation (as I usually do). I think the title says it all here. People try not to become involved, and not to meddle in things that seem not to be their business. However there are times when you have to meddle, and you can't stay neutral.
This novel seems to suggest that no matter who you are, or in which country you reside, you can not help but become involved in things. I see Fowler as "experienced" in age, yet "innocent" in his attempt to stay out of things and let them ride as they will. Pyle, is "innocent" in age and experience, but absolutely eager to get up to his elbows in things. The wisdom of Fowler which comes from experience in life, leads him to attempt to keep his nose clean and observe from afar. The inexperience of Pyle leads him to get dirty more quickly and perhaps, with experience, he will learn to regret some of these hasty actions.
It is a lesson for the world, perhaps. Some countries are too hasty to act on the events in other countries which may not share the same perspectives; yet, countries and the world are now so interconnected and interdependent, that not to action may prove just as detrimental. It's a curious problem we humans have created for ourselves.
Definitely you can comment on the themes of innocence and naivety versus experience and cynicism as expressed through the two main characters. The relationship between Fowler, the cynical, world-weary but politically astute Englishman, is obviously set in contrast to the youthful, naive and optimistic views of Pyle. In a sense, Greene uses these two characters to present two different political approaches to the world, with experience definitely winning out.