You are liable for all the rent, whether roomie pays you or not. However, you may be able to limit your damages by arguing that L knew R couldn't pay the rent, you asked L to evict R so you could get someone in who could pay, & L refused to do so. Thus, L did not "mitigate her damages" & is only entitled to the damages she would have incurred if she had acted to evict R promptly. Depends on your judge if this will work.
Here's the WA Landlord Tenant Act.
http://www.leg.wa.gov/pub/rcw/rcw%20%2059%20%20TITLE/rcw%20%2059%20%20%20TITLE/rcw%20%2059%20%20%20TITLE.htm
You MUST file an answer to the complaint by the date & time listed on it to avoid default. You should plead the defense of L's failure to mitigate damages, and plead a counterclaim for unlawful ouster/forceable entry/termination of utility services for the mailbox name incident. You can get an eviction kit with instructions & sample forms you can adapt at office supply stores, legal services companies, or from Washington Legal Blank, Inc., Issaquah, WA.
http://www.walegalblank.com/ Filing the answer will delay the eviction by at least 2 weeks while the court schedules a fact-finding hearing. Make sure you bring all your proof -- letters, witnesses, checks, etc. Testimony from the mail carrier will be necessary, unless L will stipulate that she removed your name from the mailbox. (Can you just tape your name up on the outside of the box?)
RCW 59.18.300
Termination of tenant's utility services -- Tenant causing loss of landlord provided utility services.
It shall be unlawful for a landlord to intentionally cause termination of any of his tenant's utility services, including water, heat, electricity, or gas, except for an interruption of utility services for a reasonable time in order to make necessary repairs. Any landlord who violates this section may be liable to such tenant for his actual damages sustained by him, and up to one hundred dollars for each day or part thereof the tenant is thereby deprived of any utility service, and the prevailing party may recover his costs of suit or arbitration and a reasonable attorney's fee. It shall be unlawful for a tenant to intentionally cause the loss of utility services provided by the landlord, including water, heat, electricity or gas, excepting as resulting from the normal occupancy of the premises.
The question of course, is whether mail service is a "utility." The utility list was only 'for example'. You should argue that mail service is a service as necessary and as standard as electrical or garbage service.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.