Merge Rows and Views PDF integration
Finally I solved applying this patch to views in order to get GROUP_CONCAT as an aggregate function in views UI and uninstalled views_merge_rows.
I think this is a better solution from a sql query performance point of view: views_merge_rows simply changes how data appear. Probably I was wrong in configuring something in it but I won't investigate any more.

Thanx all for reading...  ;-)