Some critical points about your installation:
- You've got a hybrid MBR setup, which is dangerous and unnecessary. It's probably best to go with either a pure-MBR setup and a BIOS-mode boot of Linux or a pure-GPT setup and an EFI-mode boot of Linux. Either approach is possible, but you'll need to use GPT fdisk (gdisk) to change your partition table. I'm not sure which approach would be less likely to create boot problems.
- You've got evidence of both GRUB Legacy and GRUB 2 installations on your hard disk, but GRUB Legacy is installed in the MBR. This is unusual for recent versions of Mint, since Mint has used GRUB 2 for a long time. OTOH, the GRUB Legacy setup does include references to Mint 14, so it looks valid and should theoretically work.
- Your USB flash drive also has a GRUB Legacy installation.
Chances are that your flashing question mark at boot time is because the firmware is looking for an EFI boot loader but not finding one. I'm a little unsure of precisely what the Mac firmware will use and under what circumstances, but my suspicion is that you'll be able to get it working in either of two ways:
- BIOS/MBR -- Convert the disk to a pure-MBR setup with gdisk (type "r" followed by "g", then type "w" to save the changes). If you're lucky, it will then start working, or at least launch GRUB, since the Boot Info Script output suggests it should work.
- EFI/GPT -- This one's trickier. You'd need to convert the disk to a pure-GPT setup by creating a new protective MBR and convert /dev/sda1 to an EFI System Partition (type "t" followed by "1" and "EF00" in gdisk; then type "x" followed by "n"; then type "w" to save your changes). You'll then need to create a FAT32 filesystem on /dev/sda1 by typing "sudo mkdosfs /dev/sda1". Then you'll need to install an EFI boot loader on the ESP. The easiest way to do this is probably to mount it at /boot/efi (you may need to create a mount point), download the binary zip file of my rEFInd, and run the install.sh script. With any luck, you'll then see rEFInd when you reboot, and it should launch Mint.
I can make no guarantee that either method will work, and in fact there's a risk that either one could make matters worse. Given the relative complexity and nature of changes for each approach, it's probably best to try the BIOS/MBR method first and try the EFI/GPT method only if the BIOS/MBR approach doesn't work. (The EFI/GPT procedure has an equal chance of working if done after the BIOS/MBR method is attempted.)