The Shroud sabotage has been revealed to me. When should I mention it, or should I keep quiet?
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.
There are plenty of outcomes here. Obviously you can cure the genophage which is all nice cupcakes and rainbows but in times of war you need to do what's hard and assure multiple allies. To that end the best thing to do is to allow the sabotage.
When you are in the final talk with Mordin you can try and delay him (wont work) or you can tell him of the sabotage. If you stick to the 'evil' plan but don't have enough renegade or paragon points you will only have two options "I'm not stopping you" or "I will stop you". If you do have enough renegade points, avoid using the renegade interrupt and instead choose "I will stop you". A renegade prompt appears allowing you to shoot Mordin in the back and prevent him from curing the genophage.
This causes the Krogan to believe they are cured and the Salarians happy because there will not be a Krogan uprising. This nets you two more armies for your fight against the Reapers.
Your first attempt to discuss the Genophage is interrupted by turbulence. Your second attempt (in front of Eve and Wreav) will generate many Paragon points.
I found when speaking in front of Wreav he says something about "remembering this". Not sure what impact that had, so I waited until speaking with Mordin to inform him, because I do not trust Wreav.
I checked the Mass Effect wiki for a more comprehensive review of the outcomes.
It lists these three options at the Shroud:
At this point, Shepard may either let Mordin go or stop him to sabotage the cure per the Dalatrass' request. There are multiple outcomes to this conversation:
If Shepard warned of the Dalatrass' deal in the convoy, Mordin deploys the counter-sabotage and deploys the cure, sacrificing his life in the process. This is the only outcome in this situation. You may use a Paragon interrupt just before Mordin steps into the elevator to get some morality points, but it won't change Mordin's fate.
If Shepard did not reveal the sabotage while in the convoy to the Shroud, Mordin will realize something is wrong. Either Shepard tells Mordin about the Dalatrass' deal or Mordin will correctly surmise it. If Shepard tries to sabotage the cure, and Wrex and/or Eve is alive, Mordin will refuse to listen. Shepard is offered a Renegade interrupt to gun him down. If Shepard does not take it Mordin will deploy the counter-sabotage and successfully cure the Genophage; if not, the sabotage is left intact, and the cure fails.
If Shepard has not revealed the sabotage and tries to make Mordin abandon the mission while Wrex and Eve are both dead, persuade options can be used on Mordin to convince him that it is for the best if the Shroud remain sabotaged and the krogan fooled into believing the genophage cured. This is the only scenario where Mordin lives. Both Mordin and Wiks agree to fake their own deaths, but Mordin becomes a War Asset at the Crucible project, whereas Wiks, according to an e-mail he sends you after the mission, instead seeks out a quiet colony world where he will work as a farmer and take his secret to the grave.
With this evaluation of the Aftermath (edited for relevance):
This mission results in the following outcomes:
The turians provide the 43rd Marine Division, Seventh Fleet, and Engineering Corps. Primarch Victus can no longer be found on the Normandy.
Whether or not you actually cured the genophage, the krogan provide the the Urdnot Leader, Krogan Clans, and Clan Urdnot. You will also earn Krogan Mercenaries if Wrex is leader. However, they may be diminished in strength if Eve died or Wreav is leader. Furthermore, if you did not cure the genophage, Wrex will eventually find out, and confront you during Priority: The Citadel III, leading to his death and a diminishing of the krogan's military strength. If Wreav is the Urdnot leader, he never discovers the deception. Urdnot Wrex/Wreav will no longer be found on the Normandy.
If you cured the genophage, you will receive no salarian assets at this time, leaving the STG force you gained from Sur'Kesh as their race's only representatives at this time. Sabotaging the cure grants you their First Fleet and Mordin Solus (if he was spared).
If Shepard stops by, Diana Allers performs an interview which depending on the mission's outcome and morality choices can result in War Asset improvements e.g. to the Krogan Clans asset.
Given that the Salarian First Fleet's military strength is 150 points while the total combined military strength of Clan Urdnot and Wrex is 330 points, with another 75 from Krogan Mercenaries, a Renegade-minded player should be aware that not curing the genophage with Wrex as leader will actually weaken your force, whereas Wreav as leader will provide a total of 475 between the two races.
If you're concerned with role-playing ethics, do what you think you should do.
If you're concerned with maximizing your war assets, it depends on whether Wrex or Wreav is the Krogan leader.
If Wrex is leader, cure the genophage. If Wreav is leader, sabotage it.
That really depends on the motivations of your character.
The first time I was prompted to mention it, I did, but I was interrupted before actually saying anything.
The second time I was prompted to mention it, I did, and...
Both Wrex (he is alive for me) and Eve were very happy that I told them, and said they would know if it hadn't worked. Mordin said that they wouldn't know since everything would appear to be fine, but that he could fix it. In order for Mordin to fix it, he had to go to the top of the Shroud while it was exploding. He sacrificed his life in order to distribute the cure.
I'm not exactly sure what happens if you do sabotage the cure and don't mention it. This question (possible spoilers in title) mentions some outcomes of that.
Well, first off, is it your intent to cure the genophage, or do you want to go along with the Salarians plan?
If it's your intent to cure the Genophage, you can reveal it whenever you want. The earlier you do, the more Paragon points you can earn along the way.
If it's your intent to sabotage the cure, then the key is to not reveal the sabotage to Eve while riding in the truck after the first battle of the mission. You can start to try on the dropship if you want the paragon points, but so long as you don't tell Eve, you can continue to sabotage the cure.
I got through this part and the way I did it was just not say a damn thing. Every body kept asking me what was wrong and I kept shrugging it off. Eventually Mordin detects a temperature change and at this point I told him about the sabotage and told him I was wrong... where he eventually asks me why, and I responded with something along the lines of, "It should be your choice." At which point he decides to sacrifice himself and thanks you for the option. which I thought was a lot better than what everyone else seems to have gotten.