Is it permitted for the cantor to add tropes or phrases such as "King of Kings" to the Lamb of God? Also, should it be sung during the sign of peace?
No. The USCCB clarified this in September 2012 that it is no longer permissible to alter the text of the Lamb of God. The number of times it is sung may be lengthened however to cover the action.
The Agnus Dei is meant to be sung during the breaking of the host. The sign of peace, which should be brief, ends and then the Agnus Dei is sung. It remains problematic that the sign of peace is often difficult to end since many treat it as a kind of meet and greet rather than a quick sign of charity to those immediately nearby. Catechesis is necessary to keep the sign of peace brief.
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