Hi
A murmur is caused by turbulent blood flow through the heart. This can be congenital (perseverence of the foetal blood flow) or acquired (eg a leaking valve). It can also occur without an anatomical cause and this is a flow murmur which means you have no pathology or risk.
Normally we would just do an ultrasound of the heart called an echocardiogram as it is cheap and will tell you the anatomical cause of the noise. A cardiac MRI is a v expensive way to get the same result.
You need to ask your cardiologist to show you the result of the MRI and explain what is wrong. Once the pathology is known it is possible to work out if there is a risk or not of incapacitation and whether your regulator is correct or incorrect. Answers to your other questions then follow
Good luck