15. How do I make sure my a Raider I just obtained is included in my lineup?

When you first obtain a Raider, the system will automatically set it to "In Battle" . If you want to change the setting, go to "Collection > Raider", select a Raider, and toggle between "not in battle" and "In Battle"

You can also quickly tell whether a Raider is "In Battle" by looking at the Raider profile.