I believe I may be able to suggest something similar to what it is you are trying to achieve.
1. True, there is no "Declare Defeat" trigger effect and this is a major problem outside 1 vs 1. However, it is possible to kill every unit a player owns in just one or two triggers. For example:
Condition: "Destroy Object" - "Henry V"
Effect: "Kill Object/Remove Object" - "Player 4" (don't specify an area or a type of unit)
This should destroy everything Player 4 has and so will put him out of the game.
One problem you may find is that if you have a scenario with a very high population or a scenario which lasts for hours the trigger will crash the game. A way to avoid this is to split the effect between several triggers. Thus you kill off the player over a few seconds rather than in one fell swoop.
2. In regards to your second point again it is true that there is no treason option in the editor. However, all treason does is reveal the location of a specific unit for a few seconds. This effect can be duplicated by using the "Change Ownership" effect. Once to change the ownership to your own (or ally's colour) and then back to the original shortly thereafter.
What condition you use to effect this is of course a matter of choice. In an upcoming scenario I've used tribute of a specific amount(say, 400 gold?) to an idle player. Obviously, the more players you want the harder it will be to work it all out. I can see that to have a large multi-player regicide scenario you'd have to be pretty damn cunning. But then, isn't that half the fun of scenario design?
Hope this is of some help.
Best Wishes,
Frostius