A. It is in fact trial and error until you get it right. Use the Reverse Dir(ection), One Way, X axis, Y axis flags of the func_door_rotating until you got it right.
B. Browse my maps and you'll find an example map on that subject.
C. Use the env_beam entity for that. Take a look at the HL Entity Guide to see how to set it all up.
D. Its done with a trigger_hurt entity. Create a bruch with the AAATRIGGER texture and turn into a trigger_hurt. Again, check out the HL Entity Guide for its workings.
E. Uh... not really sure about that one, check for tutorials and the HL Entity Guide.
F. -
G. Entity Weapon Strip.
H. Read our tutorials.