Hi and welcome to Frog Forum
I have not kept White's treefrogs, but there are many on the forum who do. Let me give you my take on your questions.
Question 1: Sir Hopalot is a cool name for a frog. I have noticed in my experience with frogs that they will do nicely without some of the frills. As long as the water/terrarium is clean, the food is good, and the temperature is right, is about all that's needed. I worked at a zoo as an amphibian keeper and kept frogs two different ways - in a very simple plastic box and one that has all the frills. Either way, the frogs were healthy and bred regularly. So preferring to sit on a rock rather than a tree doesn't seem unusual to me.
Question 2: White’s tree frogs have the ability to change color depending on environmental conditions. Temperature, humidity, light intensity, health, and the color of their surroundings all can affect what color a White’s tree frog is.
Question 3, I'll leave for the experts