Theory time, ladies, popcorn, lets go!
much is made of how much harry resembles James. JKR has laboured this point forever. it's a trick, a device. remember snapes pensive moments. How Lily hated James because he bullied Severus, but James wanted Lily, She apparently overcame that. Severus , on the surface, hated lily and despised her pity because she was a dirty filthy mudblood. bit too convenient for my liking.
The books have always struck me as being very age appropriate, rather than catering for us grown ups. they have dealt with more adult themes as the central three characters of Hermione, Ron and Harry have grown up... the world as seen through Harrys eyes has become more complex and darkr. I don't think that marital infidelity is too far stretch for JKR. I also believe that Lily is feted as the 'brightest witch of her age' for a reason - a simple charm to have Harry resemble who she thought may be a more fitting father ( wizarding world seems to be rather moral at time) given that she was married to James wouldn't be a huge stretch for her.
the only thing that she couldn't change was is eyes, resembling hers.
you wiat, 18th birthday for harry = hook nose and greasy hair, spell runs out when he becomes of age. and Snape has taken an oath with Lily that he has to protect Harry, hence not 'smiting him up big time' in the last chapters of book six, (either that or he simply LOVED Lily and couldn't hurt her flesh and blood..)
and breathe (or snigger!)