Just a wild stab in the dark (and maybe someone who knows for certain can comment), but try deleting the "ver.req" file. Maybe it's presence can prevent loading? (you don't need it anymore. OTOH, I assume you only added it when you were unable to boot it up, so perhaps this won't help.)
And also try another memory card if you have one.
The shortcut button on my s3 is the default for entering ALT mode, but it doesn't have any effect unless you first boot up CHDK off the card, which you have not yet been able to do.