Trisquel uses a modified version of Ubiquity the Ubuntu installer, which will
be why you weren't given a Debian 'completely overwrite' option.
IIRC the GRUB2 question was so that you could dual boot to propriety
operating systems without needing to know the automatically generated grub
password. If you're not dual booting you have no need to worry. If you have
dual boot then a
sudo dpkg-reconfigure grub-pc
Will let you change it to the package maintainers version (just accept the
first two dialogs).
