Oooh! Good topic!
1.) My poster would be a giant picture of: Half the Xeno's toothy face lined up with half of David's face. David would wear an impassively bemused expression and have a few subtle blood splatters on his face. The tagline would read "In space no one can hear your prayers."
2) I would release A BUNCH of trailers and I would reveal that Walter had still been alive when he got abandoned on the planet.
I would release at least one prologue in the style of "The Crossing" narrated by Walter. It would chronicle his attempts to get off the planet and save the Covenant. Shameless self-reference here, but it would basically be my fan-fiction, and it would conclude with something like "If you get this message do not under any circumstances come to this planet."
3) Well, I'm 31, am I outside the target age group? I guess my target demographic would be sci-fi/horror fans of all ages!
4) No. Because even though I know they exist, it is really really difficult for me to conceptualize anyone not enjoying Covenant. (Sorry Dr. Curt Connors, you have my sympathies, I just can't relate.) I would probably get fired. :P
5) I will have previews that show a lot of action, expressive and sympathetic characters, some implied romance and loss, and a wide array of visually engaging settings:
A very beautiful looking tropical planet at sunset (with a weird number of moons or something, maybe?), an eerily bright and sterile laboratory setting (with the camera slowly panning to some gory sight, suddenly disrupting the immaculate feel of the lab),
Ship corridors marred with blood, claw marks and littered with the mutilated bodies of colonists.
A very brief glimpse of colonists getting egg-morphed.
The same tropical world at night depicted as a nightmarish jungle full of danger around every corner, with some brief flashes of the Xeno.
6). Plenty of nods to the original movies including chestbursting scenes (with the baby xeno looking more like how it looked when it came out of Kane), and staying true to the Xenomorph's original life cycle.
7.) Which Android? I'm not sure if David should grow on people, he's a psychopath. He should continue being eerily calm and sadistic. Walter will be afflicted with guilt over his failure to save the Covenant and secretly in love with Daniels.
8.) ...This is all I can think of for now...maybe I'll think of a synopsis later.