The start on flag means it will start spawning right after the level is loaded. You probebly don't want it so untick the flag. The monsterclip flag means the monsters created by that multimanager will be blocked by the monsterclip entities. The cyclic flag means after the maker spawned once it will spawn again untill all it's monsters are gone, This has no affect since it may spawn only one monster.
So as I understand, apart from the replacemant of the monsters, you want a message to show up after the item is picked and after you pass a certian doorway, is that right?
I'll tell you what you need to do!
Give all the "prisoner"(friendly) monsters the same name. Copy that name to the items killtarget value, and now all the monsters will disapear as soon as you pick up the gun. Make the gun target a multimanager. That multimanager will target with no delay(0) the first game_message(not sure of the name, but choose the game entity, not the env_message) and maybe an ambient Generic that will play some nice little beep or some other kind of sound when the gun it picked up. That multimanager will also trigger all the monster makers, make sure they all have the same name.
For the second message you'll need to use a multisource or a trigger_changetarget, since you don't want it to work untill you take the item. Here are the two simplest ways to do so:(the second one is simpler)
1) Make the multimanager target the multisource. Copy the multisources name to the message entity's master value. After you'll pick up the weapon the multisource will be switched on and the message will now work when triggered, but wont work before the gun is picked up. Make the door target the message and you're done.
See the second way to do it, it's simpler, but alas the trigger_changetarget is overlooked by most mappers. It's a very usefull entity.
2) Don't make the door target the message at all! Make the multimanager target a trigger_changetarget. That tigger will target the door and in the newtarget vaule write the name of that message entity. When the item is picked the trigger_changetarget will change the doors target a newtarget, the message.