Here are the relevant rules:
- "Each round, 1d6 is rolled for initiative for each adventurer, monster, and group of identical monsters." [Initiatve]
- "A combatant using a weapon with a long reach (spears, for instance) may choose to attack a closing opponent on the closing opponent's number and thus attack simultaneously with the opponent, even if the combatant rolled lower for initiative." [Initiative]
- "Once two opposing combatants are within 5' of each other, they are engaged" [Movement]
- An "attack made after a running move... is called a Charge." [Charging]
- "Spears, pole arms, and certain other piercing weapons deal double damage when “set” (braced against the ground or floor) and used against a charging combatant. To set against charge, the combatant being charged must have equal or better initiative; this counts as holding an action. Both charging and charged combatant act on the charging combatant's initiative number and are therefore simultaneous." [Set for Charge]
- "Whenever a combatant kills or incapacitates an opponent with a melee or missile attack, he may immediately make another attack throw against another opponent within 5’ of the target he has just dropped." [Cleaving]
- "Characters armed with spears can attack from the second rank in melee and inflict double damage when used in or against a charge." [Spears]
Therefore to answer your question:
1) can you brace against a charge and hold it for multiple rounds? (thereby negating the requirement to beat the target on initiative if they're far enough away)
No. You can be assured that you will at least attack simultaneously with the opponent due to the Initiative rule, but you have equal or better Initiative to the enemy to deal double damage.
2) can you brace against a charge from the 2nd rank?
The intent is that "setting for charge" is a way of attacking with spears, and spears allow you to attack from the second rank. Therefore, yes.
3) if someone charges into a line of braced spears, does everyone within reach get to attack for double, or only the one who was charged?
Yes, everyone within reach can attack. Yes, anyone set for charge deals double damage.
That said, the rules are admittedly susceptible to multiple interpretations in number 3, because of the language "the combatant being charged" and "the charging combatant" suggests that only the target being charged can deal double damage. In this light, remember that if you're within 5' of an opponent, you are considered "engaged" by the opponent, even if he's attacking someone else; moreover, should the opponent kill his target, you are at risk of being attacked by the opponent on that initiative number (because of cleaving). Therefore in a practical sense you are a "charged combatant" if you are a combatant engaged by a charge. So while this is a slightly tortured reading of the rules, I'd say it's the correct one.
EXAMPLE: A party of 7 spearmen, with 4 in the front rank and 3 in the second rank, are charged by a pack of 4 goblin riders on dire wolves. The front rank spearmen are each adventurers (named A, B, C, and D), while the second rank are a group of hired light infantry. The goblin riders begin 300' away (two rounds of movement).
Round 1. Initiative rolls are: A 6, B 5, C 3, D 2, Light Infantry 4, Goblin Riders 4.
On 6, A sets for charge.
On 5, B sets for charge.
On 4, Light Infantry set for charge. Goblin Rider's advance 150'.
On 3, C sets for charge.
On 2, D sets for charge.
Round 2. Initiative rolls are A 1, B 2, C 4, D 5, Light Infantry 4, Goblin Riders 3.
On 5, D sets for charge.
On 4, C sets for charge. Light Infantry set for charge.
On 3, Goblin Riders charge. Goblin Rider 1 charges A, Goblin Rider 2 charges B, Goblin Rider 3 charges C, Goblin Rider 4 charges D. The fight plays out as follows:
A attacks Goblin Rider 1 simultaneously with 1's attack on A
B attacks Goblin Rider 2 simultaneously with 2's attack on B
C attacks Goblin Rider 3 simultaneously with 3's attack on C, and deals double damage if he hits
D attacks Goblin Rider 4 simultaneously with 4's attack on D, and deals double damage if he hits
The Light Infantry could attack any of the Goblin Riders simultaneous with their attacks on A-D, and deal double damage if they hit.