dudes, you all gotta realize that for all intents and purposes the Shi'ar are villains, and villians are not allowed to win. you actually find it plausible that realistically they can invade and subjugate untold amounts of other worlds but they cant beat one team of heroes on Earth?
all the pantheons and skyfathers were rightfully excluded for they dont live on earth anyway. they all occupy interstitial dimensions, and usually have some self imposed code not to meddle in mortal affairs. besides all those gods and demons exist outside of the time stream, so they can always turn to an alternate Earth for worshippers since thier existence seems to depened on if believe in them.
i really dont see how in a planet size battle like this one or two ppl are gonna make a difference. besides everytime aliens tussle with Earthen heroes its always a small group of them. the entire Skrull fleet has never tried to take Earth. they alwys send in covert mofo'kers, and decoys and shit. then one of thiers mess up and expose the operation and they get beat.
lastly in the MU america has shown to be the only country that has advanced military tech coming out the anus. and you know invaders are alwys gonna come to america. so this is basically gonna be the Shi'ar vs USA.