Trouble with Mission settings

Hello Folks,

Making a new arc and going with alot more options and customization than I have in the past but running into two quandries.

First, I have a collection as my main mission objective, but after the team collects the glowie I want to spawn an ambush. The problem is that the Collection has to be set as a mission complete since it is a single collection. Everything works, but the mission complete and exit button appear at the same time as the ambush spawns, allowing the party to leave the mission without facing the ambush. I could add more objectives and chain them, but was hoping there was a simple way to do this.

Second, Is there a simple way to make the boss versions of my custom group NOT appear when you have difficulty set to "No Bosses". They are appearing as Lts instead of bosses, but I would rather they ONLY spawn as bosses depending on the person's settings running the mission. Related to this: If I pull the bosses out and set them to their own custom group, do I penalize the mission as far as ticket rewards for having 2 custom groups (1 lacking Bosses, and the other lacking Minions/Lts) ?

Thanks in advance for any assistance.

P.S. If anyone knows a good map that is very "watery" I would appreciate that as well. Closest I found was a couple outdoor maps which are more "forest" than water.

Thanks Zamuel,

I appreciate the feedback.

What I am probably going to do is modify the mission objectives a little.
Currently, I switched the collection item to a destructable object (Idea being that the team needs to break into the vault for what they seek) and triggered the ambush on 3/4 health.

The team can still leave once the object is destroyed, but at least the ambush is moving towards them prior to mission complete. I may require them to exit as well, then only a portal will get them away from the ambush.