1. Things may break unexpectedly, but the offline features should work fine.
2. No, it'll just be recreated again.
3. Connect over USB and use my firmware page.
4. That's a question for someone else to answer, as I don't use Calibre.
5. I made
NickelMenu, which is included in the OCP packages. You'll probably be interested in some of the other features like invert screen, usb connection (since you say you won't be using WiFi much), screen orientation, and so on. You might also be interested in
NickelSeries if you read a lot of fiction in series or if you read a lot of nonfiction with subtitles. For the patches, you just read through the files in src and decide which ones you might want to enable. A popular set of patches are the
header/footer ones and the line spacing ones. For you, I'd also recommend the "Always show confirmation dialog before upgrading" and "Change Browse Kobo home screen link target - *" patches.
6. I don't use Calibre, but IIRC, it refers to syncing state with the Kobo.
Dictionaries: You can download them manually, but I'll need to know your firmware version to give more details.