I don’t believe there is a way to access the landscape in real time with blueprints- someone ask a similar question recently at the epic tour.
The last few things you mentioned are incredibly difficult technical challenges that have not been attempted in realtime before, like changing the course of a river in an fps game. And the reason isn’t that someone hasn’t thought of the idea yet.
You could certainly setup something where you can setup trees to chop down and build things. Or maybe even hack together an event where you break a dam, spawn in particles, lower volumes to simulate a river shrinking.