Motion functionality question

Yes, you can absolutely do this in Motion. Motion can do a wide range of animations or composting process almost the same as After Effects or other motion graphics/compositing software. The composites can be done in "2.5D" where you separate the clips on X/Y/Z and use a camera to move around them. I think that is what you are asking.

It's not really clear what you are trying to do. "2.5D" refers to taking flat 2D elements and moving them in 3D space. It might help if you have an example of what you are trying to do that you can link to to.

Matting does take duplicate layers, but that should't take up additional space on your drive. And no matter what, you have to render. Here's another example to see if this covers what you want to do. If I have a video of a room with a couch, a window and the outside. I could make three copies of that layer, mask out the main pieces (couch, wall with the window, outside). I could take these layers and offset them in Z to create depth. I could then take a camera in Motion and move it on X to simulate parallax. As long as I don't go to far in any direction I can make things look 3D. The duplicate layers don't take up extra space on my drive because the video source is only referenced for rendering. It's not until I export that the file takes up significant space.

Perhaps you should start by posting in the FCP forum about these workflow issues you mentioned. Maybe it’s something of a deal breaker for you or maybe it’s just because you are unfamiliar with it. FCP is very different from Avid or Premiere, and if you try to make it work like these editors you will end up in frustration; but if you learn how it works you may change your mind.