Yes, for every dungeon there are 4 different paths. One path is called "Story mode" and is intended to primarily let you experience the lore behind the dungeon and get familiar with boss and environment mechanics. The other 3 modes are called "Explorable modes" and are only unlocked when you've finished the Story mode. Explorable modes are different paths with different bosses in that same dungeon, they're difficult and require planning.
As for end-game events, no they're not open to low-level characters. Side-kicking only works downwards so people can go back to places they've already visited without feeling overpowered, it doesn't work upwards so people can't just skip content and go for the final boss in day 1. Now, like dungeons, events also have 2 difficulty levels : normal and elite. Normal events are what you'll be doing most of the time, but now and then there will be a huge boss fight that requires coordination and team play, that would be an elite event.
The important thing to keep in mind is that whether it is a dungeon or an event, your level and you gear will never trivialize the content. Difficulty will be what it will be and your skill will decide if you get to pass.