Start by merging to non-facing pages, then change the document setup.
The artwork seems to jump to the left on the left page only by about 12mm.... talking about 400pgs
Can you show us some screen shots, please?
It might be that you need to adjust the margin guides and enable Layout Adjustment before converting to facing pages.
And what version of ID are you using? Have you checked to be sure you've installed all the patches?