I've thought of that. And while certainly having a larger population will help, I think that the real advantage might be the fact that the Northwest would almost certainly be a maritime and mercantile power. Most likely, their initial exposure would have occurred in the 1500s from traders plying the Pacific Coast trade route from Mexico up to their homelands. When the Europeans first show up a century or two later, the epidemics are long over.
I'm afraid it does not work like that.
When it comes to disease, Native Americans had two strikes against them. One was lack of exposure to old world diseases, leading to a pack of diseases all doing virgin field epidemics at the same time.
The other problem is that most Native American populations descend from a very small founder population. So there is not as much genetic variety in their immune systems as old-worlders have.
Even with the old worlders better setup for acquiring resistances, the Black Death still managed a major epidemic in 1665 in London. And that is a single disease in a population with better immunevariety. What I'm saying here is that its going to take the Native Americans more than 300 years to get any real resistances going. And a higher population density means epidemic diseases will hit harder. Also, the climate in the Northwest isn't doing you any favours.
I don't want to post here just to shoot down ideas. We have other people for that. The issue of epidemic diseases have been raised here before, and it is a difficult one to tackle. Some options are:
Cowpox. Smallpox was far and away the worst of the diseases. If you can find a way to get endemic cowpox into the population, population losses will be reduced.
Variolation. It was known of in India for thousands of years. Difficult to make a connection though, might need independent discovery.
Contact with the old world, transmitting diseases and genes for a long time. Absolutely the most effective option, but difficult from your starting location.
Maybe a POD where a rather virulent disease mutates out of some animal reservoir before European contact? Several epidemic waves led to the development of quarantine, and other effective responses to epidemics? When European ships make contact, several are wiped out by a disease they have no resistance to, postponing regular contact?