@Skipper_of_Stones, I just started a thread that may be of help to you: Diabetes camping supplies?
Many of the questions you have would be very appropriate as a single thread: I think each will generate a lot of discussion. So, one suggestion would that, after this first thread, you go one question per thread at a time, so as to be able to get some deep and dedicated answers.
At age 4 or 5, the kid will be able to do a lot of things on his own. He may be able to inject himself if he is really precocious – or he may do it for the first time that trip (wouldn’t be stupid). In general, he will more or less know what needs to happen to him at all times – but his judgment may not always be good.
On the other hand, the uncle will be very sketchy, and will miss a lot of that knowledge. The boy will know more. It may be an idea for you to take some notes about your own understanding of things right now, and use yourself as a model for the uncle as you start learning about how to deal with D? D is a hard and complicated disease, and knowledge comes slowly. Essentially, you need to learn to think as if you were a pancreas (that’s the title for a great book about D btw, you want want to buy it).
Possibly the boy would know some things that the uncle wouldn’t believe, and the uncle would go against what the boy says because it would sound wrong.