Can Adventure creatures always be cast from exile?
Resolving an Adventure spell puts the card into exile and allows you to cast it from exile. If the card was not put into exile that way, it cannot be cast from exile. The Throne of Eldraine Mechanics Guide says this on the topic:


  Now the fun part of going on an adventure: returning safely! If an exiled card you own is on an adventure, you can cast the creature. This could be the same turn in which you cast the Adventure or on a future turn. This works only if the Adventure spell resolves and the card remains in exile. If you cast the Adventure and it's countered, you're out of luck. Of course, you don't have to cast the Adventure at all. You could just cast Beanstalk Giant if you have need of smashing now or you don't want to risk Fertile Footsteps getting countered.