Salaams sister Ma Mooli 
How do you know that any particular Mujtahid is a “naib e Imam”?
- Does the said Mujtahid appoint himself to this elevated position?
- Does the eminent Imam by his own wish anoint the Mujtahid as his Naib (deputy)?
- If it the above (no. 2) is correct, how does the eminent Imam convey this message (That a particular Mujtahid is from then on is the deputy (naib) of the imam? And how can this message from the Imam be verified? Letter of appointment, perhaps? For the Imam is known for communicating through letter writing, right?
If the source of guidance of all your “Mujtahids” is the eminent Imam, then there should be no two opinions on any matter between “Mujtahids” (as they have the infallible Imam to guide them, right?)
The late ayatollah Khomeini and his likes promoted the concept of Wilayat e Faqhi so the 12er Shias can form a government in the name of absent Imam whereas there equally very prominent ayatullahs opposed to this concept and take it to be an innovation in matters of religion.
How can the ayatullahs (Mujtahids) differ if they are guided by the same source (Imam)?