i think you did a pretty good job. you can lead players to rp, but you can't make them act. (or sometimes think)
i think one thing i might have done differently as a dm, had i caught it, was when they basically decided to ignore all their plans and just charge in. i think i might have called for some wisdom and int checks and pointed out some of the obvious - the concept that the powerful dragon didn't really try that hard on the way in, the planar thing would definitely have occurred to the characters, even if not the pcs, so i would have felt fine giving them that information for free and without player prompting (after all, players are not the characters, i can't expect them to act and behave, know and recall, the same that the characters would).... having the characters "suddenly recall / intuit" the deal with the planar portal and the details of teleporting.
and when my parties are about to make a tpk decision, i usually specifically ask them to review what they have already learned / know / the characters would realize, recap what has happened, restate their proposed solution, and make sure everyone is in agreement.
putting it all in context like that in one utterance, especially the recap and restate (sometimes i only do those two), frequently brings the magnitude (and sometimes the stupidity) of what they are about to try into context.
maybe having the dragon grab the last prying eye, hold it up to his eye, say something like, "i'm warning you..." and then letting it go, may have driven home the, "i can see ethereal" bit a little more. could have had some creative fun, like having the last eye stagger its way back, it is covered in cracks, and the playback stutters. after all, great big dragon grabbing a tiny little scrying detector is bound to be a delicate task, not well suited to such overwhelming strength.
gaming groups are like guys. for some you need a brick, for others you need a really big brick. and for those special few, you need to @THWAP them with a +20 PILLOW of EPIC GET-A-CLUE.