yes there is other .cia that works too, you must search again
if you use rxtools, you can inject fbi app to your emunand
and delete other region app via fbi app
Its happen to me too,
I think the problem is when you change region only all system is changed to europe but the language still japan.
So i use config.3ds from some leaked nintendo sdk. And change language setting to english or somethinh else. And change eula setting to ff.ff
yes i had it too on my JAP console with other region cias, no need to open app, just close and open your 3ds, it will re apper
but you still must unwrapped the app before use it.
maybe this can help
https://github.com/roxas75/rxTools/releases
yes, because if you inject secureinfo first it will search for US system file, but found nothing and your emunand will be brick (blank screen).
dont know about failed injection, but check if secureinfo file name must be "secureinfo_A" as i know
go to nintendo 3ds folder/code1 folder/code2 folder/create new folder rename as dbs/make title.db and import.db
it will ask repair if you enter 3ds software management