Undies - wash one, wear one. Why would carrying more be better?
Everything I have can be laundered in the sink which I do every couple of days. Marino wool and the synthetics are dry within hours.
Flash 18 - I do exactly that. I use it as a daypack at my destination.
My toiletries are the same as I described at the end of This Post
Yes - the list is everything I have with me - including what I am wearing.