There's no easy way out of this, and the older your playerbase gets, the more often it'll happen.
Having more players can give you leeway - I prefer to get a minimum of 4 in possible campaigns to be able to both split tasks and coordinate in case of missing people.
But the key aspect, and the one that will be harder to manage, is to not have a main character; Its easier said than done, as in theory all your players should be main characters, but if you find that a plot point hinges too much on a single person - your example of the King demanding an Orb that's carried by a PC sounds like a key example - then it might be time to reevaluate how you're treating everyone else.
If all your players have spinning plates, someone missing a session will be a lesser deal - all you have to do is go balance another event of the story with the others. But, again, like I've said - easier in theory.