Arduino Dimmer Shield Schematics Interpretation
Edited: This is a good example of a question that superficially APPEARS to be Arduino related, but all the material dealt with is 100% EE related. If people are so utterly hateful of all things Arduino (as some truly are) then it behooves them to leave the question alone for others to deal with appropriately.
The question has now made the EE -> Arduino -> EE journey pointlessly to nobody's benefit.



The interface to this circuit are via optocouplers OK1 &amp; OK2. It is a pure electronics hardware question. What drives OK1 and OK2 is completely utterly and wholly unrelated to the core question. 

The circuit is designed "strangely" and has a connection shown which may not exist in reality and which is lethally dangerous if actually present.

DANGER:

The wire boxed in orange connects AC side to Arduino side.
IF X1-2 is grounded or Neutral nobody and nothing may die. But they may. If X1-2 is Phase or live then Arduino and people may well die.
This connection must not be present and may not be in practice.



A key confusion in your assumptions is that load current pases through the rectifier bridge. It doesn't. If you take out the bridge and R3 // R3 the TRIAC will be unaffected. The OK2 cct relates to zero crossing detection and is unrelated top TRIAC function.  

The load appears to be intended to be connected between X2-1 and X2-2, as you say. This places it between the gate side of the TRIAC and one mains leg (usually neutral) which is unusual and risks confusing people who are used to normal arrangements so may be dangerous, but should work OK. 

When OK1 is turned on by PORTA-3 the gate (connected to OK1-4) is driven on via R5. T1 conducts and a mains to load path occurs via X1-1 - X2-2 - through the load - X2-1 - through T1 - X1-2

Mains is always across rectifier 1KAB to full wave rectified mains drives OK2 via R3//R13 

U$1  is a schottky diode in reverse connection across opto LED to stop it having (fatal) reverse voltage applied.

ack.imgur.com/xxR3Z.jpg