If it worked out for you in that thread you maybe should mark it solved, to suggest to other readers and the few that search for themselves that there just might be a solution somewhere in the thread. You could even add a clear summary of how it went, and whether you made it to buster or not...
Otherwise,
http://forums.debian.net/viewtopic.php?f=30&t=138748&hilit=+trusted I think I typed something relevant there...
In my experience doing offline machines a repo from a single dvd may not be everything. When I did that it was a 12GB+ repo on a usb stick. Foreign repos can be included in your local repo, details and testing is up to you. It can work.
Of course, the same could be used for an offline reinstall. Upgrading from local media when you've done it before is much much faster. Reinstalling is much faster if the technique is untested and fails, forcing the reinstall.