When should I mention the Shroud Sabotage, if at all?
Depends. Do you want to cure the Genophage? If yes, then blab about it all the time. The first time you try you will be interrupted, but still get Paragon points. If you do not want to cure the Genophage, keep mum.
Not revealing the sabotage will let you keep Mordin from dying, but Wrex will attack you. Curing the Genophage will result in a heroic death for Mordin.