I dont know a lot about DCF, but if have been a resident of the UK for 6 months, and have the job offer in the US, you should be able to file for your wife. You may have more luck with one of the UK-US sites such as visajourney.com.
Oh and yes, your wife can visit the US for 6 months, but this doesnt sound like a visit to me, so IMO, she would be lying to immigration if she entered on a visitors visa.