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Ze_German-Guy answered the first four questions in the oher thread (and I agree with him). As regards your last question then the use of Alien tech cards is mandatory in alien VP scenario, see p 16. This post actually reminded me to answer the other one.
As for question 5: There is no official way to use the empire advantages, though you could certainly use one for yourself (likely making the game easier) and even assign simple ones to the aliens.
The alien technologies are used in the official scenarios IIRC.