You say that R7 and C6 form a second phase shift. Please see the attached file(dim.png)
you know, triac is acting like a switch. So as soon as Vd1 and Vd2 equal 30V the Diac conducts and the triggers the triac. I drow the schematic and have changed the potentiometer value from zero to 500kohm. you can see the responses in attached files: dim-g0.png , dim-g50.png , dim-g500.png
As you can see in those files the responces of 2 circuits are very similar and There is a small phase shift that seems negligible. So I can't find out how R7 and C6 are improving the behavior of the circuit. Is there something the matter with it?
Regarding with the LC lowpass filter, Is it a filter in the times when triac is off? Because when the triac is on, it's a short circuite and L1 and C2 look in parallel. Am I right?