A prior question that you should ask (in order to understand the law) is what RCW 9.41.360 prohibits. Note para (6): "Nothing in this section mandates how or where a firearm must be stored". The part that you should focus on is the part that say "If A, then B". There are two crimes defined, "Community endangerment due to unsafe storage of a firearm in the first degree" (class C felony) and "Community endangerment due to unsafe storage of a firearm in the second degree" (gross misdemeanor). The distinguishing feature of these crimes pertains to the bad act that follows, namely death or person injury for the first degree crime, and discharging, using in a crime, or threatening. Taking the misdemeanor version, first, there has to be involvement by a "prohibited person", and that person does at least one of
(i) Causes the firearm to discharge;
(ii) Carries, exhibits, or displays the firearm in a public place in a
manner that either manifests an intent to intimidate another or that
warrants alarm for the safety of other persons; or
(iii) Uses the firearm in the commission of a crime
A prohibited person is "a person who is prohibited from possessing a firearm under state or federal law". There is no prohibition against storing your firearm in that manner, instead, there is a consequence (for you) if a prohibited person does certain things with your weapon.
The law is not limited to actual potential for a weapon to fire. The purpose of paragraph 3 is to specifically immunize you against prosecution in case a prohibited person gets your firearm. Various exceptions are given, such as lawful self-defense by prohibited person, lawful permission of the parent or guardian and supervised by an adult, or
The firearm was in secure gun storage, or secured with a trigger lock
or similar device that is designed to prevent the unauthorized use or
discharge of the firearm;
By the letter of the law, you have not used a device designed to prevent unauthorized use (instead, you have removed a device that enables its use). This being a new law, we cannot tell how the courts will react in case your non-operating firearm is obtained by a prohibited person who then uses the weapon in a crime (plastic guns that don't actually fire are also used in crimes). There exists limited legal precedent regarding exceptions to firearms restrictions and firearms that have been rendered permanently inoperable (machine gun laws), but your plan doesn't do that.
Until case law is created that concludes that the law "really means" that the gun must be rendered inoperable, the courts will interpret the law literally: it has to be in a box, or have an added device designed to prevent its use (regardless of whether it will actually fire). Since this is from an initiative measure, the Washington courts will especially look at the letter of the law.