Can a Washington resident with a WA concealed pistol license purchase a firearm, or do you need to be an Idaho resident?

Yes, you can. Just has to be transferred from dealer to dealer, in your state. With paperwork being done on your end.

I do believe Idaho respects other concealed weapon permits from other states the only states that do not is actually WA and OR

Concealed carry licence does not give you any privilege if you buying guns. Background check long or short gun. Rule apply if you are from neighboring state you can purchase long gun with background check and leave the store with long gun same day. All pistols must be transferred. Even if you live in Spokane. Or Montana. Same rule with private sales. Be aware of Californians:) they not allowed to touch guns.

Yes