I cannot fathom why someone wouldn't press the court by way of petition to proceed with probate if this person cannot carry out his duties; it's kinda unfathomable to allow estate to remain in limbo. Then again, for all I know, they owned all their property jointly w/ right of survivorship. (You also later indicate the focus of your interest is on a life insurance policy, which may not be part of the probate estate.)
It is unclear why your husband is calling the court to ask about this vs. an estate-probate attorney. Court won't walk people through procedure, and this isn't something that comes up on a daily basis (husband's being prosecuted for murder of spouse). You can always ask clerk to point to cases they may be able to come up with where this has been the case.
"There is a life insurance policy as stake in this situation."
If the life insurance is made payable to him, that's a matter of contract and the policy isn't part of the probate estate. If such a policy is your sole focus, then it would seem indeed a matter that the Slayer Rule will resolve. But I don't think manslaughter will necessarily count as "murder" under the law. You'd have to ask an attorney to research this question.
It doesn't make sense to me that you've consulted with an estate-probate attorney about "removing" him as executor and that the basic question hasn't been answered.
"And, as for hiring a lawyer, how much would a typical fee be for this?"
Depends on what you mean by "this" and how involved it is. An action/petition to contest the court appointing someone executor where the nominated party fights it might be very expensive. You'd have to ask around for hourly rates, since I doubt anything beyond preparing the petition itself would be something an attorney would likely agree to do for a flat fee.

