Edinburgh to Newark to Dulles, is 2:40 sufficient time to clear customs and make connecting flight

Change in layover in Dublin; it's only 1:20.

No you can't skip pre-clearance in Dublin, as the plane will arrive at a domestic gate in the USA. Do check though that it is done for this flight as it isn't for all.

If it is a sold connection then the airline has to pick up the pieces in case of mis-connection. One put together yourself or by a third party is another matter.

If you're a smartphone user, download Mobile Passport, you'll breeze through Customs, I was done in maybe 10 minutes, including walking. Faster than Global Entry.

I think you’ll be fine with Global Entry. We were through immigration and customs in less than 10 minutes recently. Customs only takes a second. unless you have surpassed the limit for purchased goods.

"Is 2:40 enough time to clear customs and get to the gate to catch the plane to Dulles?"

It should be, particularly with Global Entry and no checked luggage. But if it isn't, they will put you on a later flight to Dulles (and there should be several). Even though you have Global Entry, you may still want to set up Mobile Passport, so you have that option in case there's any delay at Global Entry (belt and suspenders, as my parents used to say).

"So if I choose the Dublin flight is 1:45 sufficient time?"

It should be. Just don't dawdle - head right to the gates for US flights (clearly labeled). I don't know if they have Global Entry at Dublin, but they do not have Mobile Passport.