To me, when you drill down and realize that for an individual some pleasures are much "greater" than others in terms of intensity, duration, and parts of the body affected, you see clearly that some specific pleasures are much more desirable to you than others. All pleasure is pleasure because it is desirable, but all pleasures are not the same in every respect to all people at all times, or even to the same person at different times, and therefore as to specific characteristics, some pleasurable experiences can be more pleasurable (more intense, longer lasting, or affecting more parts of the body) than others.
Right. We're making an assumption that the discovery of the scientist and the meal of the predator make each enjoy maximum pleasure. In as much as they are both mammals, with a common ancestor, the experiences would be equivalent as pleasure is pleasure.
Certainly, we can't demonstrate this scientifically as the scientist would have to hold off his discovery until we can get him in an MRI and getting the lion in the MRI presents obvious difficulties.
We're making assumptions in as much as we are doing philosophy here not science.