Yeah that's about the size of it. Actually I can't think of better timing to make a national emergency declaration on the wall than a RBG retirement. In fact Trump should make some other supposed controversial decision around the same time. Blitz the left. Stay on offense.
Absolutely. Do it 10 minutes after RBG reads her statement. But low-key. Just a piece of paper. Make it a fait accompli. No cameras, no press release. When Trump is asked, he just shrugs and says, "It's done." And the admin releases video of the wall going up that same day. Just get it started.
In fact if the admin wanted to play 5-D chess, they would have a cut-out file an injunction in a friendly Circuit. Then have the friendly Circuit rule in favor of Trump. Then if that gets taken up on cert, SCOTUS 4-4 preserves the lower Court.