The main quest is relatively small, in effect it will work to introduce you to the various regions if anything. Can do main quest in around 10-15 hours, if that. However even then for main quest, there are parts in the main quest set in the overland wider world where you can team up, its a mix really. Can skip main quest altogether if you really wanted too.
In regards to enemies levelling, its not quiet that simple, overland content and delves are all scaled to what is Level 50 CP 160. You start at level 1 so would be steamrolled at every turn, so what happens is a system called battle levelling was introduced. You start with more health, resources, damage etc. As you level up up you actually decrease to an extent in those elements with the expectation you fill them in with better gear / resources etc. This happens until you yourself reach Level 50 CP160 at which point there is not battle levelling and your down to your gear / skills. To put simply, while you are in a way scaled, you still still usually be stomped into the group if walking out the gate you try to take on world boss's. Besides there is plenty of content in this game, even when experienced that will make you cry at times
In addition there is dungeons which are set but can be toggled between normal and Veteran once you hit Level 50. The latter is 4 man content and some of them can actually be pretty touch!