Out of the picture apparently. Even teen Jean seems to be missing, and she could have psychically taken down the whole team of Avengers. Notice all the powerful telepaths were kept out of the battle (teen Jean, Rachel Quentin, the Cuckoos, and Emma Frost). Magik was kept out of the picture too. Polaris, Iceman, and Cable all nowhere to be seen.
With that said considering they were fighting both the villains and the Avengers the X-Men did comparatively well, since they are missing their psionics and their time warping sorceress (and their other heavy energy hitters).
You read Uncanny Avengers, right? the answer to both these is in there. It wasn't an Asgardian enchantment on that axe, (though Thor thought it was, and probably still does) it was Kang posing as Loki. and then Thor lost the axe at some unspecified point in the past, and did not get it back until Uncanny Avengers.
Except you don't. Some people use the interviews to show what was intended when some people don't get it, 'word of god' is pretty much as good as you can get when trying to explain a plot point. But I don't think they're required. I mean, the event is far from perfect, there has been some clunky bits, I wish some of the inversions had been more subtle, and the problem of the un-inverted X-Men bugs me. But to say it doesn't make sense without interviews is absurd, I mean, the story really isn't that complex, it's pretty easy to follow.
Nope. You're right about Kang posing as Loki and getting Thor to use the enchantment, but he didn't introduce the enchantment for Jarnbjorn to the Asgardian library, it was already there and Odin already knew all about it... otherwise Odin could not have known to tell Thor it was a bad idea to use it. So, it was an Asgardian enchantment, or at least one to which the Asgardians already had access (supposing it was actually from Nornheim or some such).
Which still makes a mess of the Eternals Saga in those Thor issues, as there was no reason for Odin not to use the axe and enchantment, at least not once he was laying down his and his people's life in no-holds-barred combat against the Fourth Host. Prior to that, it could still make sense for Odin to lay down the law to Thor concerning making moves that could potentially tick the Celestials off, he still had a whole lot of preparing to do for the Fourth Host... but honestly part of that preparation should have been readying that spell for use on the giant Destroyer's Odinsword, and once it was a case of total warfare (which it was in #300, Odin in the giant Destroyer is not being diplomatic but straight out trying to kill them), there's no excuse for not using his most effective option.