Sounds like you've presented your case very well. I would assume that private businesses in WA have the prerogative to prohibit weapons within their confines. So, if the owners decide on a weapons ban in the workplace, I don't know what other recourse you have other than to
1) comply (begrudgingly)
2) carry concealed with the risk of getting caught and possibly fired
3) quit and find a more 2A-friendly employer
Which you choose is obviously up to you and only you. Regardless of any opinions you receive here, you and only you are responsible for the consequences.
Good luck and let us know what you decide.