Uh, there's an awful lot to unpack in your post.

If a flight is delayed or cancelled - are you referring to the whole flight between HNL-South Pacific Islands-GUM? Or if the flight you're on between islands is delayed or cancelled? If you've read upthread, then either can and has happened - that's partly why each flight has a mechanic onboard as part of the crew complement. Generally, you can work with UA to rebook the next available flight but even pre-pandemic you're not guaranteed the next flight simply because of how often the planes flew the route.

Is it very likely the case - uh, no. Historically, again reading this thread, UA needs the planes to fly since that's how they make money.

Glad you want to experience the Island Hopper - many FlyerTalkers do as well.

You don't want to get stuck on an island when your destination is Guam - then I'd advise to skip the Island Hopper and fly directly to GUM if that's what you want to do.

David