On traps: no no no not at all!
You need to prepare a trap, and make sure it's knowledge OOC. You don't have to mention what the trap itself is per sé, but the reader must know that your character prepared one. Otherwise you'd get really crappy story writing full of deus ex machina's.
To our knowledge Stein never took any precautions towards defending himself from a psychic attack, so he didn't.

As for Martin's views on HGW? He has no idea that Stein may or may not try to prevent it. He didn't hear your conversation remember?
All he got was a small glimpse at the plans when he was in your collective minds.
If it really is a massive army of super powered flying monsters that Stein calls angels. Then Martin doesn't give a flying fuck.
He's going to let it all happen unless something directly interferes with his own objectives.
Keeping Mithias alive is one of those objectives, Gabriel isn't because Martin doesn't believe he'd be threatened.

Stein got moved because he was in an extremely weak tactical position compared to Martin and got cocky. This or something similar was inevitable.
Every character has plenty of strengths and weaknesses. And Stein stepped into a new domain and challenged Martin on Martins strongest points. That's just asking to get wrecked.

Now, the point off all this was so the rest could keep playing while Matt was gone on vacation. So please just continue, if you decide to wait 3 weeks this threat will probably die.