It looks like the shifter clamp you have is for a 1x setup (ie, a 5-speed with no front derailleur...)- you'll need a clamp with studs for 2 levers, and the domed washer. Your best bet is probably to just find another set of Criterium shifters, rather than trying to find just the clamp and the missing domed washer.
Here's one on ebay.fr, with the down-tube cable guide.... Though you may have luck finding one at a bike shop that's been around a while- the sort of shop that has drawers of random old stuff. These shifters came on 100,000's of bikes.
As far as the DT cable guide, unless your frame has brazed-on guides, you will need one. The Delrin one pictured I believe was standard with the Delrin derailleurs- there is also a chrome Simplex one, too, that may have usually come with the 'all-metal' Simplex mechs. Either will work.