Yes, there is still a risk, but it is perhaps not as bad as you may think.
The X rail is of course very stiff, so no problems there.
The Y rails themselves are also very stiff, particularly along the Y axis.
So the weak links are the rail sliders and their attachments to the X gantry 20x20 rail.
You could knock it out of parallelism with the bed if you tried, but the screws that hold the sliders to the melamine plates (and the plates themselves) are actually pretty strong.
So yes, you could rack it, but under normal circumstances that is probably not a huge concern.
The reason I am considering a single Y motor mod (other than the "gee whiz" factor) is that it should reduce acoustic noise because both sides will be driven simultaneously by the same jack shaft and because the belts can be run tighter (heavy jack shaft bearing blocks).
Maybe, maybe not, but worth a try IMHO.
Yes indeed. There is a lot of thrust available with those lead screws.
If one side were driven up while the other side was driven down, something is going to bend or break for sure.