First, the family is not in Grays Harbor Co, WA, on the 1900 cenus, but in Chehalis Co, WA.Your William was counted twice.On page 233 he is listed with the Allen family, and shown "M", which would signify married.But, on page 236 he is listed alone as head of household.And, thankfully on that listing he is shown "Wd" widowed.
I notice the 2 yr old in the Allen household was born in CA, do you suppose Minerva died in CA?The only CA state wide death index I know of, which is on microfilm, begins about 1905, but someone else may know of a different earlier source.Maybe a descendant of Milton, the 2 yr old, would know where in CA he was born, and that might narrow down your search.
Did you find an obituary for William?You never know what will be included in obits.There's the chance it may mention he was predeceased by his wife and where she died.
Have you obtained a death certificate for your grandmother?It should show the names of her parents, including her mother's maiden name.
Did your grandmother live long enough to obtain a Soc Sec card?If so, you can get a copy of the application she filled out, and it should show the names of her parents, including her mother's maiden name, and other data, too.