Lorne is a heck of a nice guy, and he is amazingly approachable and friendly. I got the chance to ask him about the Sandcrawler last weekend. It was his recollection the full-size set piece was built first. ILM then had to match the bottom portion for the miniature.
(That might explain why the bottom is just a big rectangle in overall shape while the top has all the interesting angles. The two don't really match and you can see a sort of "dividing line" between the bottom tread section and the rest of the ship.)
He didn't seem to recall the "headlights," but he did say the "chute" R2 gets sucked into was likely built from scratch to match the full-size set piece.
He did remember the treads though as he was the one who made them. He told us the story of how he brought up the idea of getting an injection molding machine for the model shop because he was concerned there was no other way to make all those treads in the limited time available. Therefore, he made just one master and the rest were then copied using the injection molding process.