Questions about a dimmer circuit
Your many questions: 


The diac allows low power dissipation in the trigger circuit and makes it insensitive to trigger current asymmetry and variations.
Use the diac circuit, or a triac with a built-in diac. 
To limit the current through the pot wiper so it is not destroyed when the pot is turned to near minimum.  
R2 and C2 are a snubber circuit which limits dv/dt across the triac, preventing false triggering due to stray inductance and perhaps reducing EMI a bit.