I've never had layer shifts - so can't help there. My prinbeds only go up and down - so maybe that's why.

The usual reason for things not printing central is because you haven't told simplify3d where the middle of your plate is.
If you look at this page of settings:


The origin offsets for the x and y axis are currently set to 0 - which is the far corner of the build plate. so that's where it will start printing.
change it to: 115 for the x-axis and 97.5 for the y axis.
Essentially the centre of the plate is exactly half the length and width. :-)
As your build plate doesn't go up and down - leave the z axis origin at 0.