You might have a case. First, take it to a personal injury attorney. They usually give free consultations. If the personal injury attorney thinks you have a case, you will need to open a probate to sue for wrongful death.
I would advise involving a lawyer. There can be tax consequences in such a transaction, as well as issues of Medicaid eligibility if it should become necessary for your parents that a lawyer could help you with.
Technically, anyone can contest a will to be honest. However, you need a really good reason to actually go through with it for a couple reasons. First and foremost, there needs to be some wrongdoing. Someone had to exercise undue influence or trick your mother or something of that nature. Generally, people are absolutely allowed to write people out of a will if for no other reason than they want to. If she just wants to give all her money to her church, she can. The other reason you need to...
Neither. The fee for a Personal Representative is must be reasonable given the circumstances of the estate: how much time was required, what skills were required, etc. The reasonableness is determined by the court. RCW 11.48.210
If, on the other hand, the will outlines a fee, or lack of a fee, that will control.
Washington inheritance tax only applies if the deceased person lived in Washington at the time of death, or owned substantial property in Washington at the time of death. It also only applies currently to the part of the estate that is over $2 million.
I agree with what my colleagues wrote and would just like to reiterate that probate in Washington is very streamlined and not something to be avoided at all costs like it is in other states.
Please have your mother consult with a good elder law attorney to make sure her estate plan is squared away.
I'm not sure why you are asking this question. If you are simply worried about this, then don't. It does not seem, from the facts that you give, that anything wrong has happened.
If, on the other hand, there is an APS or criminal investigation, you need to get off the internet and get yourself a real-life attorney ASAP.