Can development on this engine be learned from zero, or is that a pipe dream?
I found learning it and working with it quite enjoyable, especially compared to my experience with Unity where the lack of source code forced some very ugly workarounds on us. And with "only" low millions in annual revenue, Unity wasn't even willing to discuss the price for source code access... but with UE, you get a triple A game as demo project for free.
TL;DR: yes, it's possible, but it has a learning curve.
Underlying logic and systems is written as code by programmers. Tweaking and configuring behaviours is done by designers in Blueprints.
if a self taught 15yr old can do it knock yourself out.
The quality of all tutorials, the easy free download, marketplace and the community are great!