A lot of misinformation in this thread.
An MC, in the context of SAF alone, is merely a formal medical opinion. The decision to grant medical leave is the prerogative of the unit commander, i.e. the CO.
For the most part, MCs are recognized to be almost synonymous with medical leave. However, the CO still has every right to withhold/revoke medical leave if he deems fit. This is not the same as revoking an MC. If a CO decides to do so, then he will be liable for any damages/injuries that arise as a result of deviating from medical opinion.
Prolonged MC requires a medical review or board (depending on duration) to assess fitness prior to resumption of training/duties. This is not for the purpose of endorsing or rejecting an MC.
Again, it is the prerogative of the unit commander to decide if he wants the serviceman to return to camp for medical review prior to consumption/completion of the MC.
So short answer to your question: yes, medical leave can be declined by the unit commander at any point in time, whether before or after consumption.