Is editing of hotkeys possible in Opera Email
either in the UI or by editing a text file?
Text file. Opera Mail is essentially the same as Opera 12, minus a few features, so setup files are mostly the same. But the editors are missing.
Ideally you shouldn't edit the originals, but then again they are unlikely to be updating so you could. The officially-preferred method though would be to create a folder in your profile to hold a file named keyboard.ini (or whatever) and then edit operaprefs.ini to point at that file.