Battery shorts and current
To answer your first question, the amperage ratings that you have quoted are based on many factors, not just the gauge of the copper wire. So, what can happen if the current is 8 times bigger? It depends. If there is good air circulation or the wire is very short and connected to a good heat sink then you might see nothing happen at all. The wire might get very hot and melt, or it might melt its insulation, or the insulation might smoke or catch fire. The ratings are telling you that exceeding those current ratings might be dangerous.

As for the second question, you assumed an ideal 9V voltage source and then performed an experiment with a very non-ideal 9V battery. The real battery has an internal series resistance and has a limited current capability. So, connecting your wire across the battery terminals did not draw anywhere near 56A and you did not draw anything close to 900mAh from it. Therefore, the battery was not completely discharged by the experiment.