If you literally interpret WAC 232-12-051 as written, a conclusion could be reached that since the powder company does not recommend it for use in ALL muzzleloaders, then it is not legal.
I seriously doubt that is the intent of the WAC. I suspect that pellets were not even around when that portion of the WAC was written. I have never heard of anyone being cited for using pellets in Washington.
You cannot escape the possibility that some officer, somewhere, sometime, will decide to enforce this literally as you have done, and cite you. A letter from WDFW will do you no good, if the information provided conflicts with a judge's view. That's been proven on here, many times.
You could have 50 officers tell you they are legal, but the 51st may view it otherwise.
Your options as I see them are (a) go ahead and use the pellets with 99.99% confidence that you will never have an issue, or (b) use something else.