I don’t believe that particle systems can be converted into keyframe animations. You can bake simulation data in the game engine into F-curves, but this doesnt include particles.
My uncle actually had to do something rather similar to what you’re trying to accomplish right now. He was using feathers, and Maya, but the same principle applies. For the leaf you want the camera to track, you need an actual object. This is also the reason that particles can’t be turned into keyframes animations. They aren’t technically onbjects. The scene my uncle did has around 20 or 30 feathers, with the focal feather being hand animated. Every feather actually was hand animated. The feathers would rotate and spin on a predetermined path, and there were around three or so different animations. Theses paths were then just duplicated, rotated, and given a slight time offset, giving the perception that each feather is moving differently.
For you’re leaves, the focal one will HAVE to be hand animated if you want to make an easy job of tracking the camera to it. For the rest, I suggest making a few hand animated object leaves, and duplicating the paths and such as mentioned above, placing the copies behind the focal leaf, but not too far away. ext, set up the particle system to be the furthest away, ad have it emit the leaves in the style you want. This way you set up sort of three “layers” of leaves, the focal, nearby hand animated ones, and particle ones. with enough tweaking, they should all look like they’re part of the same cluster.