They are more or less cylindrical, with apparently minimal internal structure, and protection against the sand, so it seems like a sort of exoskeletal structure could make sense.
Perhaps. But I feel like all three of those are tied up in a feeling that Vader thinks he’s irredeemable. I mean he was gonna let the emperor kill Luke there for a few seconds. He probably surprised himself.
I don’t think it’s too much of a fan-wank to think that the same supernatural reflexes and split second precognition that allows a Force user to parry a blaster bolt would also allow for Vader to not take any shots at Luke that he knew with pretty good certainty Luke could avoid.
Yoda explains what he meant in the very next exchange of lines:
“Unfortunate that I know the truth?”
“No! Unfortunate that you rushed to face him… that incomplete was your training. Not ready for the burden were you.”
They latch onto “unfortunate” but it’s the same reason that it was “unexpected.” Yoda had hoped to train Luke fully, then break the news that Vader was his father, then send him off to confront him when he was prepared. Or at least train him fully and make sure he was prepared before confronting him, so that if Vader revealed the truth then, Luke would be able to handle it. Vader torturing Han to provoke Luke, via his Force-connection to his friends, into leaving Yoda too soon is what was unexpected, and unfortunate.
As for Luke being raised on Tatooine, with the Skywalker name, two things come to mind:
maybe Luke’s hiding being so “obvious” was a bit of a decoy. If Vader did find him and kill/take him, “there is another” more hidden Skywalker, who would be protected by the fact that Vader would think he got his only heir, and hence wouldn’t look for Leia.
Given what we see in ROTJ, I’m convinced that Palpatine was pretty disappointed in Vader being maimed, and was playing the long game, hoping that Luke would take his father’s place. It’s entirely possible that Palpatine knew all along where Luke was and kept that knowledge from Vader, either through some sort of Force mind control/memory altering, or simply giving him orders that always kept him away from Tatooine. (This becomes even more likely if you bring in evidence from TPM and ROTS that Anakin’s birth could be the work of Palpatine and/or Darth Plagueis, making his bloodline that much more important, and his maiming that much more of a setback to the plan.)