[Suicide Junkie]

Blanking out the AI_designcreation.txt should prevent the AI from changing any of your designs...
You will just have to be sure that the AI's build orders are consistent with your designs' names and types.

The politics will be much harder...
Wars are easy to encourage, peace will be problematic.

Clearing the research queue will not help, I believe. Eliminating research centers from the game will.

It depends on the particular area you want to affect. For details, ask an AI guru.

Reccommended steps for creating a scenario:
1) Create a mod that vastly increases research and resource production (100x is good)
2) Start a new game normally, with all players under human control. Add one extra race.
3) Have each race research whatever you want them to start with, while the extra race researches EVERYTHING.
4) Have each race colonize whichever planets you want them to, and have the extra race do all the stellar manip you want done. Have the extra race build and gift any super-advanced tech ships you want the players to start with.
5) Abandon all of the extra race's planets, and scrap all their ships, killing them. Do nothing for a turn or two until the logs empty out.
6) Save the game.
7) Look in SE4\scenario... duplicate the .TXT files in there, and match the new names to your savegame.
8) Edit the duplicate .TXT files, and write up your scenario descriptions, and message windows.
9) Ask forum goers for more details on specific items if required.

PS: Any method of getting to step #6 will work fine.
One of my scenarios was simply based on a singleplayer game that had an interesting turn of events.

[ October 16, 2002, 15:43: Message edited by: Suicide Junkie ]