So if you make a desciple of war you don't need to go back and make a separate descliple of the land character to craft you just need to reach rank 20 and then you can begin crafting with say a pugilist class?
Uh... You can cahnge your class on the fly, just equip the required weapon or tool and you're now this class.
You do not need multiple characters in this game, you can change your class from the very beginning on.
This has nothing to do with it, it's about class quests, a questline with a story, cutscenes and all that fancy stuff. You can start this questline when you hit Rank 20. Every class has it's own Questline which opens up if you hit Rank 20. This has absolut nothing to do with Guildleves. It's like the main story quests, just for a class.
And yes, it opens up at Rank 20, everyone who says otherwise is a liar or is confusing it with Guildleves. And you need to be Rank 20 in the specific class. A Rank 20 Archer can only start the Archer Questline, not the Mining one. But as soon as you hit Mining 20 too, you can start this Questline too.