Let's assume for a minute that the scenarios are possible and that the aggressor can do the maneuvers I am proposing. Let's also assume that the aggressor has Freedom of Movement and would not be affected by an adverse conditions himself and can perform the combat maneuvers on the opponent (no size, 10 legs, etc).
1) What happens to an opponent that is simultaneously the victim of a Bull Rush and a Trip?
2) Does being Prone have any affect on a continuing Bull Rush?
3) Does being Bull Rushed "downhill" have any affect on the victim?
4) Does being Bull Rushed through an area with another effect trigger that effect?
- for example, being Bull Rushed into a Grease area, would the victim have to save or fall as if moving w\o an acrobatics check?
Here are the Bull Rush rules:
Bull Rush
You can make a bull rush as a standard action or as part of a charge, in place of the melee attack. You can only bull rush an opponent who is no more than one size category larger than you. A bull rush attempts to push an opponent straight back without doing any harm. If you do not have the Improved Bull Rush feat, or a similar ability, initiating a bull rush provokes an attack of opportunity from the target of your maneuver.
If your attack is successful, your target is pushed back 5 feet. For every 5 by which your attack exceeds your opponent's CMD you can push the target back an additional 5 feet. You can move with the target if you wish but you must have the available movement to do so. If your attack fails, your movement ends in front of the target.
An enemy being moved by a bull rush does not provoke an attack of opportunity because of the movement unless you possess the Greater Bull Rush feat. You cannot bull rush a creature into a square that is occupied by a solid object or obstacle. If there is another creature in the way of your bull rush, you must immediately make a combat maneuver check to bull rush that creature. You take a –4 penalty on this check for each creature being pushed beyond the first. If you are successful, you can continue to push the creatures a distance equal to the lesser result. For example, if a fighter bull rushes a goblin for a total of 15 feet, but there is another goblin 5 feet behind the first, he must make another combat maneuver check against the second goblin after having pushed the first 5 feet. If his check reveals that he can push the second goblin a total of 20 feet, he can continue to push both goblins another 10 feet (since the first goblin will have moved a total of 15 feet).
and Trip
Trip
You can attempt to trip your opponent in place of a melee attack. You can only trip an opponent who is no more than one size category larger than you. If you do not have the Improved Trip feat, or a similar ability, initiating a trip provokes an attack of opportunity from the target of your maneuver.
If your attack exceeds the target's CMD, the target is knocked prone. If your attack fails by 10 or more, you are knocked prone instead. If the target has more than two legs, add +2 to the DC of the combat maneuver attack roll for each additional leg it has. Some creatures—such as oozes, creatures without legs, and flying creatures—cannot be tripped.
and Prone
Prone: The character is lying on the ground. A prone attacker has a –4 penalty on melee attack rolls and cannot use a ranged weapon (except for a crossbow). A prone defender gains a +4 bonus to Armor Class against ranged attacks, but takes a –4 penalty to AC against melee attacks.
Standing up is a move-equivalent action that provokes an attack of opportunity.