Why didn’t he go further?
If Ben has been judged by Smokey then this means he has also been judged by MiB. So why didn’t MiB go further into the temple and where does he reside before he flows out of the grate?
Did MiB have to be there to witness Jacob being killed? Why didn’t he send one of his own followers -assuming he has any- to kill Jacob thus making his journey to the temple quicker and giving less time for Jacob to infect Sayid, which is my own opinion.
Concerning MIB being there to witness Jacob being killed; I believe the answer is yes, MIB had to be there. Only the leader of the Others can request to go see Jacob. Locke was now the leader of the Others, so he had the right to demand Richard to bring him to Jacob. That is why MIB chose to replicate Locke!