Legal Question in Wills and Trusts in Washington

I think that your question is actually 2 questions: (1) Can a lawyer, who prepared the will for a person who has since died, act as the executor (now called personal representative in Washington) of the deceased person's estate? (2) Can the lawyer who prepared the will for a person now deceased, then act as the executor of personal representative to oppose an heir's contest to the the validity of the will?

My opinion:

That appears to be a conflict or conflicts of interest that would not be permitted by the court and/or the bar association. There are other considerations that may also apply, but your factual statement would have to be fleshed out to determine what other issues there are.