PEX is allowed for water services here.
It should have 24" of cover. Seeing as how it's PEX, 1-1/4" sounds about right.
I've never seen the ground freeze more then 10" deep around here. The worst was in Mill Creek, farther from the Sound and some of the hills South of Bellevue. Those are about 1,000 feet high.
Farther away from the Sound and the lakes, it gets much colder in the Winter.