1. Yes.  Look at me! I didn't take the GAMSAT last year and get back from the US late January, still taking it in March.  Barely studied before I left too.
2.  You could do honours and take the GAMSAT again, or take a year off and take it again...I'm not sure 'failing' the GAMSAT is the right way to put it.  Also, you can take it in September so if you're super concerned about how you go in March you could try taking it then. Or, if you're uncomfortable with going away in December (not that you have any need to be) you could take it September 2013 and then March 2014 to make up your two tests.
But presuming you do go into other employment...
What kind of major would interest you? Something else in the biological sciences?  If you stick to the med prereqs plus a practical subject (eg Techniques) you leave a lot open. I liked the second year Micro stuff but it's not for everyone.  In terms of employment without a higher qualification I'd be looking at Pathology and Micro stuff so you could go into lab work.  If you are considering postgrad aside from Optom then maybe think about what you'd like to research for a PhD or Masters.  Cell biology (like developmental biology) and Neuroscience are both interesting but less 'practical' fields.
By the way, you don't have to take Physiology for Optom? I feel like, because you said three subjects and only mentioned four, it's meant to be on the list maybe? :3