[Dimaz]

1. Assasination spells work immediately after they are cast, so if you target enemy commander without teleport abilities or with teleport abilities but not on the lab (and not equipped with boots of planes or gatestone), your assasination will happen, and it will happen before any magic movement related battles. If you target enemy commander on the lab with tele abilities, you have essentially 50/50 chance to initiate assasination (depends on unit IDs and the orders flow direction for the current turn). If the target has boots of planes, I think he will always move first if he was ordered to cast teleportation from boots (but here I may be wrong).
2. All magic assasinations will take place before any magic movement or magic summoning ordinary battles.
3. If you teleport force A on enemy prov and move force B there, you have several possible outcomes:
A destroys enemy, B moves to your province to join with A.
A loses, B moves to enemy prov to fight remains of his forces.
If enemy also moves his forces, they will join after fight A if you lose, or attack your combined forces if you win and he moves from neighboring province or has flying. If he ordered move from not-neighboring province and the force in target prov was destroyed during magic movement, the movement is cancelled.
If you send Horrors or GR on the same prov, they will fight independently of your A force but definitely before B moves in. So, in this case it's possible to have A fight with defenders, then GR/Horrors fight with A. If they win, the prov will become independent and your B force will take it without fight. There is no way to ensure that GR/Horrors will fight enemy and not your army, or at least I don't know about it.
4. This is somewhat tricky, at least for me. Some observations: if you attack from stealth and teleport some force in, the stealth force will appear during the magic battle. If you attack from stealth and cast GR/Horrors instead, you can end with attacking full enemy force with your stealth force, or just capture the prov if GR already cleared opposition (not sure here, haven't done this for a while). Personally I prefer moving stealth force from neighboring prov which you *don't attack remotely* this turn, this way you will definitely capture the enemy prov if the GR/Horrors succeed in clering opposition and the enemy not moving something there too from neighboring square, which is actually a good counter to this type of raiding.
If there was no magic battles, stealth units will join ordinary fight or attack alone, and I think it'll happen after ordinary movement, so you can't "ambush" the enemy without magic movement/summon "trigger".