Is there any relatively painless way to import an animated GIF into Photoshop (CC 2015) and then be able to export a functioning GIF file back?
You probably mean an animated GIF, is that correct? Because you can simply open a single layer GIF file just like any other. For that matter, you can simply drop an animated GIF file on Photoshop and it will open all the layers. You can do the edits you like and then export it as a new animated GIF file.
I hope this answers your question.
Doing File → Open Will Work, just open your gif, and it will open as layers, and you can edit the frames using the timeline. (Window → Timeline)
By the way, I'm on Photoshop CC 2019.