It's even better when you guide them into finding the fatal flaw for themselves.

Very well said. So many engineers balk at "coming off as positive" as a form of lying or as a pointless social ritual, but it's the only thing that gets you a seat at the table. Engineers who say "no" or "that's stupid" are never seen as leaders by management, even if they're right. The approach you laid out here is how you have _real_ impact as an engineering leader, because you keep getting a seat at the table to steer what actually happens.