Hi Pamela, I might have found more information for Henry Williams, Susanna and Posey's son.
I found a Henry Williams and family on the May 1885 Washington Territory Census in Klickitat Co. Census. Henry, b. OR, abt. 1863, married, farmer; Theodosia, wife, b. Wash. Terr, 20 and Nellie E., b. Wash. Terr., 37 months.
In 1887 they are in still in Klickitat Co., Washington Territory. Henry, b. OR, abt. 1859, teamster; Theodocia, b. Wash. Terr, 23; Nellie E, b. Wash Terr., 2 and Baby, b. Wash. Terr, 2 days.
In 1900 Henry and family are living in Lookout, Skagit Co., WA. Henry, b. Jan 1857, OR; Theodosia, 34; Nettie E., 15; Oren L., 12; Roy L., 10 and Harold L., 4.
In 1910 Henry L. Williams and family are living in Pine Forest, Klickitat Co., WA. Henry L., 52; Theodosia, 41; Roy L., 20; Harold L., 15 and Alfred P. 7.
I'm not sure if this is your Henry or not.
As you know Henry died 1913 in North Yakima, Yakima Co. You can request a copy of his obit from the Washington State Library, www.secstate.wa.gov/library/ask.aspx . They will search the newspaper microfilm for 60 minutes at no charge & email you what they do or don't find.
I'd also contact the Yakima Valley Genealogical Society, http://www.rootsweb.ancestry.com/~wayvgs/http://www.rootsweb.ancestry.com/~wayvgs/ who does research. They might have him listed in the Polk County directories, scrapbooks, etc.
I didn't find anything about Posey or Susanna's deaths. I'd contact the Clark County Genealogical Society, http://www.ccgs-wa.orghttp://www.ccgs-wa.org and see if they have any more information. They may have indexes of cemeteries in their County and will know who to contact in the County about old land records. Successful Searching! I did find death info for Posy [their spelling] Williams.