Simple answer:

The AG returned back the Addendum to the palace because there is no House Arrest law in our federal constitution. So, unless a bill is passed in the parliament to amendment the federal to include a notion of  house arrest law, only then a house arrest Addendum can be legally implemented.  

One needs to know how POWERFUL the federal constitution is. 

Even his majesty is bounded by it. His executive powers are exercised in line with what's written in the federal constitution.