How does washout increase static pitch stability in subsonic swept wings? How does washout increase static pitch stability in subsonic swept wings, more specifically in slow moving aircraft (eg. ultralights), 15mph to 60mph? 

I understand washout is used to delay full stalls ( at least in unswept wings), but isn't pitch stability related to cg and centre of pressure?

See picture below of a european designed Archon SF1 rg ultralight with swept wings and retractable landing gear. Mid engine single prop is shown forward of the twin vertical tails.
Since its earliest days during the pioneer era, washout on a swept wing has been accurately described as "putting the tail at the end of the wing." It works in exactly the same way as a tail stabilizer.

As long as the change in moment of the rear surface is greater than the change in moment of the forward surface the plane will be statically stable. It matters not whether the tail is inboard, outboard, moved ahead of the wing (as a canard), tacked on the end of a swept wing, or even tacked on the back of a straight wing in the form of a reflex trailing edge.

On a straight, unswept wing washout can delay the stall a little, but stalling is a complex subject and best treated separately.