Doctor Strange would have been able to reveal the truth to Tony Stark, but here’s why it has not done so !
This is a question that you are inevitably posed : When he saw the 14.000.605 possible future, even if Doctor Strange would have survived Thanos according to some theories, why has he shared his findings on their single victory with his teammates ? Would it have changed things ? 1. No, it probably would not have changed anything, see worse. 2. As confirmed by Joe Russo, one of the directors, there has simply not had the opportunity. During an interview with Collider, he responded simply : “He died. This is why he does not tell them. He gets a kick up the ass in the meantime”. A reason all in all simple and effective, if one excluded the fact that super-heroes have still found the time to mount a plan of attack against Thanos. Except that Doctor Strange should be doubt that admit to Tony Stark why he spares his life by giving the Stone Time to the Titan would necessarily be the course of their “victory”.
This is one of the fundamental principles of time-travel. If you know what will happen in the future, you will inevitably make different decisions that if you didn’t know. You might even find yourself with a future more disastrous than that of which you had knowledge. Well in Avengers : Infinity War, this is probably the same principle. Doctor Strange knows that it is necessary that Tony remains in life for the only future where they win is achieved. Let’s trust in him on this point, his decisions are rarely one-sided. Now that we have more or less solved the WHY, it remains to answer the question HOW ? And that falls well, Tom Holland (Spider-Man) and Karen Gillan (Nebula) you talk about spoilers and time travel, anything you need to give some tentative answers.