North Sea endurance versus fleet-keeping speed

Could the Black Swan class operate in the North Sea, and why were some escorts still left behind from Scapa Flow?

Yes; the limiting issue was often keeping up with a fast fleet, not simply surviving North Sea conditions.

Black Swan was built under a service expectation that ships should operate in the North Sea and South Atlantic. Surviving those waters was a different requirement from maintaining fleet speed through them. Smaller destroyers could accompany Hood on routine operations yet fall behind when she accelerated toward an engagement such as Denmark Strait. Larger Tribal-class destroyers had more scope to maintain that pace. Escort size also had to accommodate armament and top weight, while fuel consumption reduced the weight carried low in the ship during a patrol.