Your Linux Mint can only be chainloaded after grub-install succeeds.

I wouldn't pay too much attention to the Grub2 warning. It is a user's right to specify where the boot loader should be but putting it on a logical partition does have reliability problem if the partition table is altered. I haven't done a lot with Grub2 but my latest Ubuntu is in sda16 and it is controlling the MBR until I recently formatted its partition by mistake. I do run Grub2 on floppy as well as on CD.

In my #164 post I did mention you might have to grub-install it again if it fails. I could not explained why except I suspect a bug inside Grub2. It happened to me before.