I don't have any advice for you, but just glad you are doing your research! Here in CA it is real tricky too. You can't even kick out squatters, who have no contract of any kind, once they move their possessions in! If WA is anything similar, whether or not your verbal contract is valid would have no bearing. I guess when they say "possession is 9/10 of the law," they really mean it!
Here are some links that might be helpful, if you have not already seen them.
http://www.lawhelp.org/documents/159...ateabbrev=/WA/
(talks about evictions on page 15)
http://www.tenantsunion.org/rights/