hmm my experiences of the sigma 70-300mm are that its macro feature is where the lens really shines (well it does its best at least

). I would guess that you shot these handheld and that I think is where you are getting some problems.
I would advise getting a monopod if possible (since I think you might not be allowed to use a tripod in the exhibition - though you could ask) to give you more support under the lens and lessen the chances of image shake.
From there I would set the lens to the macro settings and then turn the AF off - yep turn it off - and focus manually. That will allow you to put the point of focus where you want it to be (mostly on the eyes with such shots) rather than have to let the AF choose or just be on centre mode. When I shoot macro I tend to move my body very slightly rather than adjust the focus - that way I can stay at max magnification and still get shots.
Lighting - as said - is a key factor though you can invest in a something like a speedlite 430 or 580M2 and a diffuser and get some very good results. The flash will also be a good solid investment allround for photography as flash has a lot of applications.
As for settings (yours seem to waver quite a bit - experimentation is agood thing though so keep it up) you could try the following:
Aperture f13
ISO 200
Shutter speed 1/200sec
I use those settings most when doing macro with my flash and it gets good results. Boosting ISO can help with lower light and I would try to avoid using wider apertures since you lose out on the depth of field - which means that you have to be more creative in how you position the camera to make best of the little depth that you have.
You have made a good start with macro work - keep it up
