You’re not missing anything! Large-scale terrain changes were not something I was thinking about when building out the map system. Adding and removing blocks and setpieces isn’t currently supported by the trigger system. I’ve been thinking all day how I’d now go back and add it, but it’d be some pretty major retrofits. Most likely approach would be adding a trigger that can add/clear a certain kind of setpiece to/from specified tiles. I’ll make a note to explore that but it might be a while.
For second issue – I recently added functionality to the “emit trigger signal” event where if you feed a bunch of units into its “array” slot, it will emit that signal for each given unit. This is somewhat crude, but does allow you to then have another 1-unit trigger to respond to it and e.g. add invulnerability. There’s not a good example of it in-game yet and it hasn’t been thoroughly tested, but that’s what I’d try.
Sorry I don’t have more answers to offer beyond that atm!