Can saline solution mimic the look of fillers in the cheek area so I'm not stuck with it if I don't like?

Saline solution can be injected in the area that you would like to get fillers, and it can give you a close idea about what the results can be. There are many filler options and they have different indications and they may look different once they are injected.

This is a novel idea but you have to remember that the fillers are viscous solutions and their physical properties give the characteristic aesthetic outcomes that you see following injection. It is somewhat complicated to explain but every type of filler 'class' has optimal sites of treatment - some are used for the lips, others lip only and tear troughs for their biophysical properties and Tyndall effect and so on ... Your idea is a good one but unfortunately is not practical and will not likely give you proper feedback as to what it will look like.