Grek: Unfortunately, it would mean making the algorithms work differently to keep checking upwards like that. Currently weight only moves one tile at a time, and moving the weight back up would probably cause very weird interactions with the downward weight distribution. I can't look into it now, but if I get a chance I'll get back to you.
Merlon: Thank you. The weighting for collapse strategy you proposed will work most of the time. However, sometimes the cube that gets removed will cause a cascade failure. You can simulate this yourself in the program by getting a cave-in state and removing all the highlighted tiles (or shifting down the column, as appropriate). It'll sometimes leave new highlighted tiles. Though, personally, I like the idea of using cave-ins to engineer caverns carrying a risk of complete failure.
Draco: The weight distribution of above layers does effect what is possible. This is intentional, as without it doing that you can do some very ridiculous things. For example, if above layers had no effect, you could support a gigantic room on a massive pillar, and then have another large room directly under the pillar. ...Without the force of the pillar above it destroying the entire room.
Fieari: I agree. I tried to get as close as possible to the 7x7 rule with the material I used for that example. A more complete approach would have different properties for different materials. Hopefully the devs will like my algorithm enough that thinking of properties for all the different materials will be worth it .