It's been a while since I used the Jatex pantsuit, but I seem to remember that you loaded a basic clothing item and then applied various poses to customise it. It was rather confusing, which is probably why it's been a while since I used it. :) There almost certainly was a decent readme with it, if I remember rightly.

Most people complain about the plastic look of Poser renders, not actively seek it out. :) I'd start off by experimenting with the speculars. Further information might be dependent on what version of Poser you're running.

To get the mannequin with missing limbs effect, you would need to either re-model the figure, or make a prop which would cap off the open end.

I seem to remember seeing a mannequin add-on for one of the Victorias, but I can't remember where it was. It might be worth your while scouting around the stores - I'm fairly sure it was a paid-for item.