The benefits of coumadin may or may not outweigh the risks.
The problem is that "good medical practice" prevents a physician from suggesting an alternative, even though he may want to. The PATIENT has to tell the physician he wants an alternative. Only then can the physician tell him about alternatives.
Normally, a patient with a mechanical valve is prescribed coumadin, so the physician is following what is called "good practice".
There is a government database called "PUBMED", on the internet.
Within this database are mortality studies involving various alternatives, including aspirin alone.
I would consult this database and you will find that there are a number of medically reasonable alternatives. Copy the studies. The datbase has mostly abstracts, so you may have to purchase the entire article.
There are other pay databases available at medical schools, if you have access.
I also have a mechanical valve and have taken coumadin for almost twelve years. I understand the side effects and don't like them either. However, for most people the benefits of coumadin outweigh the risks - without it you really could get a clot and possibly a stroke. This is not something to play around with and I personally wouldn't try to replace it with any natural products.