Temporary solution: don't ask the installer to install grub ?
I told the LMDE installer not to install grub and it worked OK.*
However, I had another OS installed already booting w/grub, so it's just a matter of adding 4 or 5 lines to menu.lst (or grub.cfg) to add a new distro.
(grub2's "update-grub" has screwed up several times, getting partitions all confused; I think it needs "--recheck")
*the grub package *should* be installed, just not the bootloader and the files it creates, and you should be able to do the chroot thing, or something like:
$ grub-install --boot-directory=/mnt/boot /dev/sda
then see if you get the grub.cfg file.