The fact that the gems in the Belt of Deltora can't be taken outside Deltora's borders makes for an interesting problem, and I think most of us have thought about this once before... This potentially means that the ruling king or queen can't leave the land whatsoever, at least not without an heir to wear the Belt at home should he die. And a ruler usually have to leave one's land at some point to go on political, maybe diplomatic, missions.
Star of Deltora spoilers!
When Adin became king, he began to supress piracy in the Silver Sea, but that must have been done remotely, by sending ships from Deltora to the sea. Unless he had one of his five children wear the Belt at the time. Remember, it does not need to be worn by the next in line. Just an heir, any heir, will suffice. That is what I think were revealed in The Shadowlands anyway, unless my memory is failing.
Marilen wore the Belt when Lief was away in the second series. However, now that I think about it, we were never told if the Belt's power worked during that time or not. If the Belt cannot shine for someone who is not the apparent heir, that potentially means that, if the monarch have to leave the land, the Shadow Lord or some other evil force will be able to invade while they are gone, and there is nothing to stop it whatsoever. The Belt would not work for the next in line because the current ruler is still alive. There is another possibility: That the Belt will not shine for an heir when it has shone for another heir. But this unlikely. However, remember the Belt gets its power from the trust in the person wearing it. Nobody knew of Marilen while Lief was out looking for the pieces to the Pirran Pipe, so theoretically the Belt would have been powerless, right? By logic you can't trust someone you don't know exist. Do you think that Rodda thought about this? Do you think I am wrong in this speculation? Or do you think that Deltora might actually have been in a slight danger?
Oh, and then there's the other heirs that's descended from Ballum, but I don't know how they factor into anything. You just need to be an heir, and not the next in line, from what we've apparently been told. But, well, they're there. I am just aknowledging that.
Very interesting ideas. I personally believe that since the belt is based on the trust of the people. As the long as the people have trust in their ruler the belt will work. Marilen is interesting since no one knew she was wearing the belt, but maybe since the people trust Lief, his trust in Marilen allowed her to fully use the belt, even if it wouldn't shine for her unless Lief perished.