Special Rules:
King of the Bridge: If an objective is contested, and one of the contesting warriors makes a close combat attack against an opposing contesting warrior that causes 5 or more damage with a single attack, the enemy warrior is “knocked into the river” and removed from the game.

Lets say you have an Avatar in this situation. If and opposing player where to target the horse with an attack that dealt 5 damage, would the horse be “knocked into the river” or the whole calvary unit? If this was to affect only horse and not the rider, than you could not knock the horse in to the river until you did so to the rider per the Dark Riders Subfaction rules (text below), because the mount cannot be eliminated before its rider is eliminated.

Now what if the Avatar is wielding Phoenix Lance (text below) and an attack that causes 5 damage is successful, would Phoenix Lance negate the being “knocked in to the river”? Putting the Avatar at its starting marker.

Dark Riders Subfaction
Cavalry units from the Dark Riders subfaction follow the same rules as other cavalry units, with the following exceptions:
• Riders and mounts are matched by their collector’s numbers, not their mount symbols. For example, warrior #12la can ride only mount #121b.
• Riders from the Dark Riders subfaction cannot dismount or be dismounted.
• A Dark Riders mount cannot be eliminated before its rider is eliminated. If three skulls show in the stat slot of a Dark Riders mount that still has a rider, instead of eliminating the mount, turn its combat dial to the starting marker.
• When a rider from the Dark Riders subfaction is eliminated or removed from the game, remove the rider’s base from the mount’s base slot. The rider remains in the mount’s saddle. • Riders from the Dark Riders subfaction can use their subfaction abilities while they are part of a cavalry unit.

Phoenix Lance
Only warriors that can become riders can wield Phoenix Lance.
If the wielder would be eliminated, instead turn it combat dial to the starting marker. Remove Phoenix Lance from the game.

Sorry but I could not edit my question so I will reply with the last part of the question.

Last but not least, would Charge/Mounted Charge be a single attack or two separate attacks?

Eliminated and removed from the game are not the same thing. If an effect is triggered upon elimination (Phoenix Lance) and the text reads remove from the game it is not eliminated. This would also allow the mount to be removed before the rider.

C/MC would count as two seperate attacks.

Thanks for the clarification. I thought what you said would be the case but I wanted to post this and make sure. The Avatar / Phoenix Lance combo would be way too powerful here.