WD1D9443 Is a first-winter Heuglin’s Gull (dark underwing). Mew Gull is a medium-sized gull with narrower wings, slender bill and slender appearance, more buoyant flight than larger gulls.
WD1D9489 Is indeed a Heuglin’s Gull (note orange legs, white mirror on P10 only).
WD1D9612 Indeed a Heuglin’s Gull in the front (dark mantle, orange legs). At the back most probably a Mongolian Gull (smaller, round head with small, dark eye, and light mantle, pinkish legs). Nice for comparison.
WD1D9638 A large gull with heavy bill, so not a Mew Gull. I’m not entirely sure with this one picture, either Vega Gull or Mongolian Gull.
WD1D9994 Looks like Slaty-backed from what can be seen from the photo, yes.