1. if your principal doesn't agree, then there is nothing that you can do.
2. you need to take a break of atleast three months to be eligible for transfer for medical grounds.
3. yes medical certificate by govt doctor is sufficient
4. you should apply at the starting part of the illness.
5. please use this only if you are genuinely ill....otherwise you just end up spoiling case for other genuine medical cases as ICAI starts looking at these things suspiciously.