im no expert, but just finished my grow with excellent results!
i got a cooltube towards the end of my grow...i used a 600w hps. i was told to get a 6in inline fan...4in wouldnt move the air fast enough for that type of bulb.
put the fan (pulling air from left to right) at point b...just after the bulb...these fans are better at pulling air than pushing. somebody else might correct me here, but one 6inch fan will be enough.
get the foil ducting and some tape (i got silver foil tape which worked good)
dont think about using any other type of fan (pc ones etc) as they just aint powerful enough. dont know how much cash or the size or your grow area...but i will need to get a good shade for next time. Just a thought as i find the light isnt spread very well with just the default shade i got with the cooltube.
dont know about number 4, but i wouldnt think it was needed