After looking at it, I went to mission properties and cut briefing duration time down to 10% of what you had in seconds and then was able to spawn after the cut short briefing. What that tells me is there may be a limit to how long the briefings can be. What that limit is I don't know, I've never made one longer than 30 seconds or so.
I didn't see any cue card events, but that may be a moot point, because when I tried to add one, cue cards don't seem to be working (that has happened before) Your only alternative may be to use text instead.
Good luck!