I went to the top of a mountain in the first phase (where there is a statute of a man and a woman in a loving embrace) and I found a trail near there and I broke open the wall (with aard) and could peer down into the cavern and saw a treasure chest, but I couldn't go into the cavern from my vantage point. So my question is whether this is the same cavern under neath the waterfall, or is this a different one and if so where is its entrance?

There is a pretty room with a swimming pool down below, and a treasure chest, but I can't jump down there.

It's not the cave beneath the waterfall, and you get to go in that room eventually. You'll know you're near that point when you have a choice to take Triss there with you or go there on your own. For reasons which must be obvious, bring Triss.

"Singing bushes...jabbering stones...elven hogwash!"

Thank you -- I'll be patient until the opportunity arises and I won't spend more time looking around for an entrance at this phase. Greatly appreciated!