There are very many barriers of this sort.  Since  the  All-Wise  Qur’an  speaks  with  all  mankind,  it  mentions  what  is apparently a particular incident, and recalls all events similar to it. It is from this point of view that the narrations differ concerning the Barrier and Gog and Magog, as well as the writings of the Qur’anic commentators about them.

Furthermore, the All-Wise Qur’an switches from one event to another distant one due to  the association of ideas. The person who  fails to  think of this association supposes the two events to be close in time. Thus, the Qur’an’s predicting the end of the world from the destruction of the Barrier is not because the two events are close in time, but to make two subtle points connected with the association of ideas. That is, the  world  will  be  destroyed  just  as  the  Barrier  will  be  destroyed.  Also,  just  as mountains, which are natural divine barriers, are firm and will be destroyed only at the end of the world; so the Barrier is firm as a mountain and will be levelled to dust only at the destruction of the world. Even if it suffers  damage from the assaults of time, it will mostly remain intact. Yes, the Great Wall of  China  is one particular meaning of the universal meaning of the Barrier of Dhu’l-Qarnayn  and has been standing for thousands of years and is still there for all to see. It is read as a  long, petrified, meaningful line from ancient history, written by man’s hand on the page of the earth.

YOUR THIRD QUESTION

Brief replies concerning Jesus (Upon whom be peace) killing the Dajjal are found in both the First and the Fifteenth Letters, which should suffice you.