The house in Parverhill is used during an event and can't be opened otherwise. The door in the tower of ash in front of the treasure can't be opened in general. The text is ment to explain that you can't get it open even with Sandra's lockpicking skill.
No. Only the one in Parverhill can be opened by Chiyo during an event. This one in Tower of Ash will never open no matter how high Chiyo's magical knowledge is. I believe it IS just a decoration, at least for now.
