1. I am not sure but I think that formatting (i.e., installing a new
filesystem) necessary means losing the encryption in place on the older
filesystem. Here is the Trisquel documentation pertaining to full-disk
encryption: https://trisquel.info/en/wiki/full-disk-encryption-install
2. Besides the Web browser (Abrowser) that is proposed in the latest version,
you can enable the backport section of Trisquel's repository:
http://packages.trisquel.info/belenos-backports/
3. Security updates usually reach Trisquel's repository a few hours or days
after they reach Ubuntu's. There will indeed be a ton of updates to install
after installing Trisquel from its ISO. You can choose the NetInstall though.
You would then minimize the amount of (unsafe) software you download.
4. You can install from a USB key:
https://trisquel.info/en/wiki/how-create-liveusb (I would simply use 'dd'
from your Debian system)
