I think most replies here are valid especially squirting some penetrating oil regardless of position of the screw. I think Moen will give suggestions too so you might want to call them back and ask for advice tho I doubt it will be much different than here. Or think of it this way.... you got so many replies here and most are telling you similar advice so can they all be wrong or right?
One other thing you should think about.... if you mess up the screw after trying to get it out, what will you do then? You might want to leave this to a plumber to begin with. Yeah I know you are trying to save money but will you, if you mess it up? In other words, don't be penny wise and dollar foolish (and no, I'm not a plumber). And if you do hire a plumber, watch and learn and maybe the next time you will know how to do it without asking here or calling a plumber again.