I think you are in the early stages of battery failure. One of the physical results can be battery swelling.
The battery extends directly under the trackpad.
There's not a huge amount of free space inside your MBPro. If some piece takes up more space than normal, and it's directly under the trackpad, it doesn't take much of a stretch to think that the swelling battery will apply pressure underneath the trackpad, with two results: The trackpad button may not have room to move properly, and the trackpad surface itself may not be as sensitive to touch, or may respond erratically, because it's TOO sensitive.
Anyway, it's a typical result of a swelling battery.
The battery case should be completely flat (and does not rock when placed on a flat surface)
The newer MMPros, with "form-fitting" battery, would probably not have the same issue, as they are not fully enclosed in a hard battery case, and would have some "squirm-room" and likely would not have the same physical issues when the battery ultimately begins to fail.