For Q1 Q3:
Please check if you add
#import "Common/ShaderLib/GLSLCompat.glsllib"
to your shader code.
Our engine already give a solution for “One code all GLSL” 
and with it we shall use varying-attribute instead of out-in for compatibility with GL2.0.
Q2:
please check this thread , hope it may help.
BTW:
use App.getRenderer().getCaps() check GLSL versions supported