So here i try to give you a bit of help with what little knoweledge i have:
1) The middle girl Skirt and tie doesn't look right cause because they are too glossy/reflective, the downsampling have nothing to do with this. So try to play around with those sliders in the surface tab: Glossyness, Reflective, metallicity and metal flake.
2)The lines on their forheads again aren't caused by downsampling but by the hair cap. Try to mess around in the parameter tab to get it inside the figure head (usually i scale it's sze from 100% to 99/98 %
3) sorry but here i have difficulties myself and as you said you know the basic concepts of what is a good lightining i wont mind a little help here
4)I personally use Gimp which is free and have a tons of plugins for your needs
5)They are gorgeous and well proportioned at least IMO, so don't know what you mean with this. Maybe we should see their personality and how they interact between them since in there might arise some difference in perception between male and female. But aside from that everything looks good to me