The task here is to push your fingers through the sand and into the side of the razor, thus pinning it. Once held in place, use the other hand to dig it out. You may have to be patient if the foot is anchored. Just hold on like you would with a nightcrawler, in the same position, and it will eventually relax. The foot is the food part and is very tender and tasty raw.
Mussels are everywhere.
The conventional wisdom is to collect the blue ones from areas of clear running water and not from the saltmarsh edge. It's your call. They're all food. An easy way to pick up enough of these smoky-flavored shellfish is to wade along a channel edge at low tide and just pluck them from the rocks. Another method is to use that quahog rake to reach down from your skiff and hook a few up from the bottom.