Well.....this is a complicated picture that you have painted and I would say that you have a Myxoma arising from the wall of the left atrium and involving the Mitral Valve and operating would involve removing the tumour and replacing the mitral valve at the same time. This doesn´t sound very optimistic, but in fact the results of this operation are quite acceptable in terms of heart surgery.
Yes, I have to be honest and say that without operation, you do stand an outside chance of emboli (clots) detaching themselves and lodging in more distant parts of your body.
However, you Cardiologist will be well aware of this and is balancing the threat of emboli against the danger of surgical intervention at this moment.
If I can be of any more help, then please let me know.