Use a text editor, Notepad or whatever. The exact format is json (I believe), which is ASCII (text) though in recent years they removed the excess spaces and linebreaks so it isn't quite as easy to read as it used to be.
Well, you're right. I had it set though by having already disabled the Chrome PDF viewer in opera:plugins. So I was surprised when it started Acrobat. I liked that Opera never started an outside program. Anyway, I'll use the builtin viewer and download when needed.
Default data files are different.
...\AppData\Roaming\Opera\Opera for 12
...\AppData\Roaming\Opera Software\Opera Stable for 43
There are no options for changing these during installation; only for program files.
If you mean the routes according to the post, you just have to make a shortcut with those routes and switchs.