It's true, unfortunately GMC doesn't send official confirmation for PLAB 2, until you pass. There used to be a separate PLAB visa, not sure if it is still available with the way the UK constantly keeps changing her immigration laws.
If you could show them the official letter stating that you passed PLAB 1, with your IELTS, that might help. Also, how did you pay for the exam? Confirmation from a UK bank (original) might swing them to your favour. Tell your host to send a official copy of his passport (he can stamp it in a post office), but everything else should be original to give you the best chance. If you defer your coming here, you will not be able to collect a refund from GMC. You have to prove 3 main points, 1, that your host knows you and is willing to cater for you, 2, that he is willing to accommodate you (or you could book a hotel), and 3, that you are able to maintain yourself without recourse to public funds.
It might also be best to get your certificate of good standing once and for all from MDCN, you'll need it when you pass - plus your housemanship certification.
All the best. I hope you have the materials for your exam.