Well you can always try to call the dealer and see if they will do anything for you, its worth the call. Has ford ever done work on the plugs, maybe you can blame them for something they did a while back.
I personally would not repair the alluminum head, I would scrap them both and buy some upgraded ones if you got the spare cash. If not, I have read NOT to use the Hellicoils for repairs and to only use a Timesert kit (that comes from SVTech76 on here).
You can get a new replacement head for around $350, adding gaskets you are at around $400.
Then you could have someone do the install or try it yourself, check with your local race shops and see what they say, maybe they have a deal on a set of upgraded new heads.
BTW, plugs being blown out are common on the 4.6's, so maybe if you get a nice person / manager at the dealer they will give you a xmas present and warranty it for you.
~K