I have a 2011 GTI LTD 155, but the battery is the same.
As you well know, that battery base is way down there. I went in head first and didn’t think I would get out.
If it is just spinning, 2 things to check. 1, did the nut fall out of the bottom of the base and the bolt is attached to nothing. See if it will pull up. 2, assuming no such luck, you will have to capture that nut or you may be forced to cut that bolt with a dremel. I think they are stainless, so that makes it even more difficult. Try to hold the bolt up with some force, wedge a screwdriver maybe between the case handle and the base to try to recapture that nut.
Suggestion: Before you put the battery in, remove the base and run bolts in from the bottom. Jam nut or Epoxy them. Then you have studs that the battery case can slide over. A nut on top cinches that case down instead of bolt.
Sorry, man. Not fun.