If I understand your theory correctly, the reason why Shift+Preview doesnÂ't work is because the cutScene is scripted directly in the init.sqs, right?
If this is the case, IÂ'm sorry to disagree with you. I often start short cutscenes in the init.sqs (they are good for hiding stuff that needs a little time to load before when the mission starts, like fire scripts and other stuff).
Shift+Preview=Briefing works for me in these situations, but I have encountered the same problem occasionally. I have another theory...
You need to hold Shift for a few seconds after the editor starts to load the mission. I donÂ't know exactly how long (it probably depends on how fast your processor is). If I release the Shift key too early, the editor seems to skip the briefing and start the mission. But as I said, itÂ's merely a theory of mine :-\
In other cases, though, there are some commands that donÂ't work well in the init.sqs, unless there is a short ~0.01 delay before them. IÂ'm drifting towards offtopic land here, but it seems to me that OFP executes as much as it can from the init.sqs when the mission is starting (in editor as well as in SP/MP/CAMPAIGN-mode). ThatÂ's why I think you just need to hold the key a little longer when you are in the editor.
HereÂ's some examples of commands that can affect the contents of your briefing page...
disableUserInput true is the real killer. OFP will open the briefing screen, but the command will prevent you from doing anything except alt-tab and quit the game from Windows. I sometimes use disableUserInput for a few seconds if I want the player to keep looking at a certain spot upon mission start.
0 fadeSound 0 also come into effect directly in the briefing if itÂ's included at the top of the init.sqs (preventing the use of onBriefGear and similar sounds)
0.01 fadeSound 0 works, though. Then you will hear the sound in the briefing, but it will fade out upon mission start (good if you want to play music at low volume when thereÂ's a chopper near the player)
deleteVehicle unitName and [unitName] join grpNull commands is also executed when the briefing is loaded, unless they are delayed. But for some reason it seems that [unitName] join groupName is not. ItÂ'+s just OFP I suppose. Buggiest game engine released commercially, but still irresistable ;D