Rainer can pick any lock that does not say “this lock can not be picked”. You just need to try to pick it a lot. With trapped locks it might be useful to use lock picks but it’s really not necessary.
**SPOILER FOR THE CRYSTAL**
So, this crystal usefulness kind of depends. At the end of the Dji-Fadh dungeon you will find a being that you can promise the crystal to. They will give you some useful information in exchange for the promise, but as far as I can say it’s not actually necessary to give them the crystal.
However, later in the game you can find another person who is bored out of their mind, and you can give them the crystal in exchange for a nifty throwing axe. There are other music crystals in the game so it’s as you like.
