I debated if this should be a bug or suggestion or wishlist. It's kinda all. And if the UI is confusing, that can be considered a bug.
I'll justify it with a buggy behavior.
Go to a Custom Arena, go to "Staged Missions" | "Select Missions" and select a mission from the list.
Now drop down the Axis or Allied Mission combobox. Note the panel is behind the "Schedule Mis" button. That looks like a Z-ordering bug.
But now lets talk about the broader issue. You told me once that each side can only ever have one mission right? Is that still the case? If so, why is that a combobox if it will ever only have one item. It would seem confusing that the type of control suggests a choice could be made, where there is never any choice. I'd suggested a read-only text field. (Though I could see some advantages to multiple missions per side. But if that isn't in the works, a combobox doesn't make much sense.)
The "Launch" buttons beside those, I'd maybe rename "Launch AI" because it seemed confusing to me and I was thinking I would launch when those were clicked. But it is only the AI that get launched.
The "Load Mission" and "Download Mission" seem confusing. Maybe relabel to "Initialize Mission" and "Download for Offline"?
Also, I think you could just remove the "Close Button". The state it puts you in is not particularly useful when in the "Staged Missions" section. If you jsut want to go back to the map, you'd just use the "Back" menu item.
$0.02.