I think cracking would be a problem before long with anything but special cement/plaster. It looks like the refractory cement is the way to go.
Wonder how many skulls you can get out of a 25-pound bucket?
Most of the tombs in the valley of the kings were dug into the mountain side. Blocks were only used to seal up doorways then they were plastered over. So the entire thing doesn't have to be block. You can make flats out of Masonite and 2x framing and paint them to look like a tomb. Just be sure to do the ceilings also as most nicer tombs had their ceilings painted also (google "Ramses 6 tomb". Tuts Tomb is small and doable for a garage and there's plenty of reference material online.
Not there yet but I can see it off in the future. I'm a little younger than jdubbya but the whole thing hits me a little harder each year. I'll always do something but it would definitely be scaled back, since we only do a big one-day thing and then take it all down that night.
A drop panel has been our best scare for the money. One thing I've noticed over the years is that people never react to things the way you expect them to. Something that was really expensive or took a lot of time barely gets any attention, while a prop thrown together at the last minute scared people from going in (a tomato cage groundbreaker with a clown mask, windbreaker and irrigation boots).
Dollar stores have lots of great plastic parts. Also take a good look at things before you throw them in the trash. Lids from sports drinks and laundry detergent can look good if painted right, as can the bottles themselves if cut off. Most of the stuff you'll see is either brightly colored or neutral. Try to picture it steampunk painted.