yes i think you can apply for a spouse visa, maybe not a good idea thou, but if you've wed b4 the judge decides, then he could ask the embassy to issue her with a spouse visa instead.
i would appeal, but i would make sure the evidence i submitted would not lead to a refusal, as mentioned by a previous poster, i would get a solicitor to write a letter, stating he has contacted the caterer and the document supplied are genuine, and also your other evidence is genuine concerning your planned wedding and get him to certify them docs, just give them solid evidence they cannot argue with.
as for your g/f not mentioning you, i don't think it's enough reason on its own to refuse her an app, its just another reason to put down with the 'fake' caterer excuse. i don't think it asks you on the visa form if you have a relationship with anyone else in the uk, maybe your g/f didn't think she needed to mention it, as she was not asked
if your evidence is good enough, then the a case worker at the embassy might over rule it, so you could have your visa pretty quick, if not then you would have to wait longer for the AIT to decide. but with the right evidence i think you got a good chance, as something like 66% of appeals are won
