>>>Surely this feature wasn't removed, right?
That is right, you still have this feature in the fluid Pro.
>>>Please help me understand how to get Fluid Pro to give me this functionality RATHER than having lightboxed images that don't load full-screen without clicking the full-screen button that switches me to a new browser view.
You actually have this feature and much more. With the unique URL feature, you can also link to another page and open the lightbox in another page. Or you can simply Click on the thumbnails and open the lightbox, this is as you mentioned is the sole purpose of having a gallery. the Lightbox also have much more features than what we used to have in Fluid 2.x