Mechanical Monsters at the Fall of Gondolin?

"I think that mechanical dragons are a big enough detail that he would have mentioned them, even in the compressed Q30 account."

Aiwendil,
This is where you and I disagree. He couldn't mention mechanical dragons in a sentence precisely because it was a big enough of a deal. You just can't drop something like that on the reader without proper follow through.

Take a somewhat absurd example of Turin and Niniel. Imagine if he wrote in the Silm about their wedding, and then added a line "but little did either know that they were siblings", and leave it at that, and up until that point there has been no explanation about her previous life before her forgetfulness.

He would never do that, because that would be horrible writing, leaving too many unanswered questions that irk the reader. In the same way, imagine if he followed your advice and made a one sentence mention about mechanical dragons. Suddenly, the reader (who we must assume hasn't read the longer FoG) would have all kinds of questions. Were there iron dragons used before? Were they used again? How did they come to be used in this battle, and why? Why are they better for this battle than flesh and blood dragons? Were they animated by magic or was someone inside operating them, etc etc... You either have to give this concept its proper due, or don't mention it at all.

Your one sentence change isn't sufficient to answer all or even many of these questions, esp. because the mechanical dragons are tied up with Maeglin's treachery. They are a necessary plot device in the greater, more detailed story (esp if Morgoth already knew where Gondolin was), and the Maeglin factor explains why such mechanical creatures appeared exactly when they did and why.