That's pretty messed up. As a former elder I know how the elders work and it sounds like they messed this one up.
One word: HIPPA
If it were me, and I know your wife already told them that you took blood - but I would still deny it. Then I would use the two witness rule to your advantage. I would send the CO and the service department a certified letter, denying that I took blood. I would CC the elders, but I would not direct my letter to them at all. I would also say that their whole botching of this process and trying to ruin your family life in the middle of your major health situation has truly turned you off - so you will not meeet with anyone. But that for the record, you are claiming that you did not take blood. Your wife was mixed up - and you vehemently deny that you took any blood. You hope they respect your right to privacy now and that if anyone tries to inquire of your medical care, they will be in violation of HIPPA laws and you will seek legal advice to personally obtain damages from every elder and the Watchtower society in general for violating your rights.
I can tell you right now this would have an effect - it would be unlikely they would go through with the DFing - at least not right away. Should buy you some time at least.
Best of luck.