The script that you based yourself on was for 90 degree rotation through the following command (converted to JS):
myRotatedPDF.Pages(x).merge2(myPDF.Pages(x),0,myRect.Top,90,1);
For 180 degree rotation, you not only need to change the number of degrees but also the origin of the rotation:
myRotatedPDF.Pages(i).merge2(myPDF.Pages(i),pw,ph,180,1);
So your page was being rotated, but outside the visible area.
As for JS within Workflow, we have decided against updating it to a more current version because of the technical limitations of the aging Workflow development environment. The sheer amount of work and the risks associated with it are just not worth it.
However, our Automate product, which is currently available in Tech Preview mode, uses Node-RED technology and therefore offers the very latest version of JavaScript. While it is not yet a full-blown alternative to Workflow, it can already handle the vast majority of tasks that Workflow can.
You can follow its evolution (and try it out for yourself) by visiting the Automate category in these forums and reading its initial post.