Yes they are allowed but they get very busy.
Yes, but don't count on it during peak season or over long weekends. Also note the office hours: https://koa.com/campgrounds/lynden/
I think they're allowed, but they are a very busy campsite. I think your odds of getting a site that way are pretty bad. At a minimum, I'd recommend you call ahead to see what availability looks like.
Thanks! Your answer is awaiting moderation.