Thanks for doing the math, I think this is valuable even with a basic model.
I think this analysis undercounts the impact of spreading the virus. You only model local community transmission, but if the epidemic doesn't burn out in the Seattle area, it'll cause additional community spread elsewhere, which leaves room for another lg(8B/4M) = 11 doublings.
A linear reduction in R also might be too optimistic. In the plausible worst case, R is nearly constant until the final few generations when it saturates and starts to burn out.