What are the extra components in this dimmer for?
This dimmer circuit allows better control over power to the load than the basic circuit.  The same circuit can be found as figure AN1003.12 in this PDF file which seems to be written by sane people. At least, they measure their resistors in ohms. It's an application notes for triacs from Teccor Electronics published in 2002.

The extra diodes and resistors work the charges on the capacitor in such a way as to eliminate the hysteresis effect.  This is when a light dimmer is first turned up, nothing happens until a certain threshold, then you get a medium-brightness glow from the bulb.  The dimmer can be backed down for a dim glow.  But coming up from dark, you can't get striaght to a dim glow but must go up to that theshold.  By fiddling with the charge on the capacitor at the right times, this effect is reduced.  I don't totally get it, but suffice it to say that it's not fruitful to think of those diodes as a bridge rectifier, and that an explanation can be found in the PDF.