what is his reason for doing so?
It has not been revealed yet. Though it is likely to come out towards the end of the arc.
What is his canonical name?
He is using and is mostly referred to by others as Juha Bach. "Yhwach" may be a title rather than a name (basing this on the similarity to "Yahwe" - the Hebrew name of god), or a different pronunciation of the same name (in a different script/language).
Though it appears that Juha Bach is a mistranslation of Yhwach (which appears to be the official name), as @Zeno commented.