Why can the shaliach ha tzibor say birkat ha kohen in the beis of oval?
Answer:
The general custom is that while there is no actual Birchas Kohanim in the bais avel, the Shaiach Tzibbur says the text of birchas kohanim which is said when there is no kohen. This is because while the custom is not to do Birchas Kohanim, in fact if the kohen was called up he would be obligated to do so even in the bais avel.
Sources:
Dagul Mervava to SHulchan Aruch SIman 127.
