"Shaded Viewport Options" greyed out, plots are all hidden

The viewport may have been set to "hideplot" "on". It's a subprompt under the MVIEW command.

If you're plotting from a layout, you can't set shade plot from the plot dialog. It is, as you wrote, greyed out. The option to set it from the plot dialog is only available if plotting from the model space tab. I believe this is because shade plot options pertain to individual viewports, not the entire layout.
To set shadeplot for a viewport when working in a layout, select the viewport, right-click, and select "shade plot" from the context menu. With the viewport selected, you can also set this in the "properties" palette.
In the help files, it's written that you can set shade plot options for a layout through page setup manager > modify. This would imply that a page setup can be defined and saved with a particular shade option. But I haven't been able to do this. The option is greyed out just as it is from the plot dialog.
Shade plot and shade mode are totally independent. You can have the model shown as 2d wireframe but still plot a viewport as hidden, rendered, etc.
Pete

He wants the wireframe, Peter.

And?

I thought you might have missed that...