"This is a mistake, a terrible mistake!" Jokes aside, point number 1 is where it went wrong. You need to set up a targetname key for the target_level_change first. Then a deathscript that runs a script, called for example changelevel. In its most basic form it should look like this:
//(BHVD)
use ( "<your target_level_change targetname>" );
Put it in the scripts folder, and if you have everything set up correctly, as soon as you kill the NPC, it will trigger the script, which will use the target_level_change and head on to the next mission. Not sure about that error, though. I've read it would happen due to brush entities, but it seems vague. About the behavior states you listed, it's a bit of a mystery for me. I noticed them and tried to use them a lot of times, but apparently they're simply unused or not coded, or anything. Similar behavior states can still be achieved through scripts though. As for your last question, yes, recompiling the map using any of those build commands will devastate the map as the BSP -meta does, due to losing all texture alignments, lights (so the map would be plunged in eternal darkness), etc.