Is Robert E. Lee's profile carved onto the back of the head of Lincoln's statue?

No.  No such carving was done intentionally, but the myth persists to this day. The fact remains that several visitors claim to find all sorts of profiles within the tufts of Lincoln's hair.

Is there any significance to the number of steps?

No, this is another popular myth. The Lincoln Memorial steps actually extend to the Reflecting Pool, so at present count that would be 98 steps.  If one counts the steps from the road to the Memorial chamber, one would find 57.  There is no significance to either number.

Is Lincoln signing his initials using American Sign Language?

No, this is yet another myth. The artist studied casts of the former President's hands to get the proper appearance. They were both in a closed shape for the casting, the artist decided to open one up a bit to give a more life-like aspect.