I find the whole Line backup situation really frustration as well, it's a massive fail on Line and Google's part. To add insult to injury adb backup has recently been market 'deprecated' (https://www.xda-developers.com/adb-backup-and-restore-depreciated/), so this whole approach might not longer working with the next version of Android. Doing the backup has become harder with every new version of Android, I am considering to switch to iOS over that.
I think you were only looking at the table structure, not the actual table data. I also find that somewhat counterintuitive with the SQLite tool. You have to right click on the table and select browse the data, to see the content. On that note, if anybody tries to merge other tables (e.g. chat) the table structure can be different even if you're using the same line version. I believe that is a result of Line upgrading/changing a table over time vs just creating it new. This makes it a little harder to merge the data, e.g. you'll have to name all rows you're using. This is not relevant for the setting though, has that table only has 2 rows.I found the key entry and it is listed as TEXT NOT NULL, but I will just swap that out when I get the db from the new account after transferring...
One thing I should have mentioned is that Line says, you should register your email & password before transfering. I'm not sure if that is still needed, but better safe than sorry.On the new phone I am going to install the same newer version that was on the old phone and then downgrade to the same as I did on the old phone. I haven't tested restoring the line.db on my set up, but as long as I downgrade it should be OK I figure. Thanks again and I will update on how it goes.