The silhouettes are most likely them. Some food for thought:
Walkers do not follow each other in a perfect line like that, or at least not as long as the shadows did. Just as well, they would likely not stop in near perfect symmetry. Finally they certainly wouldn't walk fairly well like that(the Crawford guard in episode 4 does but the odds of two of these being in the same area and right next to each other is slim to none), they'd probably be dragging a leg or at the very least slouching.
As for what you tell Omid & Christa to do, sure they might search for Lee if you ask them to, but they're not going to spend the rest of their lives looking for Lee, eventually they'll decide that he's dead or isn't in Savannah or something and leave(Clementine as well).
We don't know how much time has passed between the ending and the epilogue, but it could have been long enough to convince them to move on.