Welcome to the site, bayfire66! I think you'll find that people here are friendly and there is a lot of good information available. We generally have monthly lunches on the third Sunday of every month; this month's lunch is in Everett (information here
Thanks for sharing your experience in Denny's. I think you handled it well and made the wise choice of leaving. While this is an open carry state, private property owners still have the legal authority to ask you to leave the premises. Failure to do so is trespassing and could result in your arrest. Once you are asked to leave, the safest option is probably to leave immediately and take up the issue later by phone, written correspondance, or arranging a meeting in person. Of course, this applies to concealed carry, too. If you were concealing but your cover garment rode up or you otherwise became unconcealed, the manager still has the authority to ask you to leave.
That said, follow-up with corporate is a good option, because most often larger chains such as Denny's have a policy to follow state law, which in Washington covers both open and concealed carry (as you know). Often, misinformed managers do not realize this and choose to create their own policy or confuse employee policy and customer conduct rules.