The BASIC version of the manner in which lift is developed never covers the symetrical airfoil because the math gets somewhat extreme.
I'm trying to avoid going into calculus here...
You have part of the idea with the "variable semi-symetrical airfoil" though. With the AOA incrasing, a larger amount of the LE is above the "line" separating the upper surface airflow from the lower surface airflow. This does give an asymetric airflow pattern.
There's also a downward deflection of air passing under the wing. The conservation of energy law will show that the air being forced to move downward means something else had an upward force given to it... that was the wing.
The best way to look at it from the modelers perspective: It works. The models fly just fine.
If you want/need more than this... you probably need to get some aeronautical engineerig textbooks. (maybe a degree in aeronautical engineering..) I used to read thes texts for fun.