Because it is much better to have an attorney - someone who can navigate through the legal process, procedure, is familiar with the court system, etc. Even a lawyer shouldn't represent themselves in a criminal case because you need someone that can take an objective legal approach to your defense. A criminal charge is not something you want to take lightly, so that is why you should have a lawyer handle it.
You may disengage with an attorney at any time for any reason, even if you had signed a retainer agreement. A simple call or email telling him that you do not wish to use his services is sufficient in your case.
It is possible but you should meet with an attorney specializing in this area and go through all details and show the attorney all medical records if you have them. Most attorneys do medical malpractice cases on contingency so it's worth it to you to meet with one or two attorneys to discuss your case.
You should consult an estate attorney. There are a lot of issues you will have to work through here. Was the estate probated? Was there an executor? Was there a will? If there was a will leaving you property of the estate, then it shouldn't matter whether you are his child or not.
There are a few more details that may need to be known - such as was there a written contract? If not, what exactly was the understanding between you two? You may have a case - you should sit down with an attorney and explain all the details and show him/ her any documentation you might have.
I would not suggest relinquishing your right as co-executor. His plan makes no sense and only leaves you not getting what you deserve. What is supposed to happen is that you sell all assets in order to distribute the funds. If he is unhappy that's too bad for him. I would suggest you get an attorney to assist you if that's possible for you.