Taqlid

Quran says: Ask those who have the knowledge if you don't know. فاسألوا أهل الذكر إن كنتم لا تعلمون Sura Al-Anbiyaa, verse 7. Obviously we can not ask a dead scholar no matter how high was his knowledge.

Mujtahid must be alive to understand the exact situation of any matter then deduct the Islamic rule of it from Quran, Sunnah, Aql and Ijmaa'.

Sunni Muslims follow dead Shaikhs like Abu Hanifa, Shafi'ee, Malik and Ibn Hanbal. Even Salafi or Wahabi sects follow Ibn Taymiyyah. All of them died before many centuries and never witnessed nor knew about our current issues. Following dead scholar does not fulfill the need of guidance at all because full guidance needs full knowledge about the situation which is with a living eligible scholar not with a dead scholar.

All Hadeeths asking us to refer to the scholars never asked to refer to dead scholars but living scholars who can be asked and who can reply after understanding the situation.

'Wassalam.

In every field of knowledge, reasonable persons follow the highly knowledgeable experts in that field. It is the logic of life that the less knowing must refer to the most knowing. It is in medicine, engineering, law, etc and no one objects on it at all. Why some people doubt it when it comes to religious knowledge? This doubt is illogical. Allah is Quran ordered to refer to those who are more knowledgeable.(Sura Al-Nahl, verse 43 and Sura Al-Angiaa', verse 7 and Sura Faaturm verse 14).

Following the verdict of the Marje' is the way for people to follow the authentic evidence derived by the highly learned experts in Islamic knowledge. Those who doubt Taqleed do not have any other way to know the evidence because they themselves are not experts in Islamic knowledge. Some of them follow less knowledgeable persons instead of following more knowledgeable which is type of injustice.

Wassalam.

No, unless you were following that deceased Marje' before his passing away and the living Marje' allows you to refer to the late Marje' whom you have followed before.

Wassalam.

We should respect all the truthful Ulama who are guiding people to the Way of Ahlul Bayt (AS) being the real way of the Prophet (SAWA) which is the Way of Allah (SWT). Even if you don't follow his verdicts and follow the verdicts of other Marje' of Taqleed, must keep respecting the A'lam. If the A'lam issues an order (Hokm), you must implement his order in any case. Verdict is binding all who do his Taqleed, butHokm (order) I binging every one including those who do Taqleed of other Maraaje's.
Wassalam.

According to majority of our Shia Ulama, the most learned Mujahid and Marja' of Taqleed is Ayatullah Sistani. There are many other big Mujtahids but he is been accepted by majority of Ahl Al-Khubra (Experts scholars) in the Hawza as the most learned Mujtahid.

'Wassalam.

You need to follow the witness of two of more of Ahlul Khubra أهل الخبرة 

Ahlul Khubra are the high standard of Ulama in the Hawza who know the level of knowledge of the Mujtahids and define who is the most knowledgeable among them.

'Wassalam.

Taqleed is based on the logical necessity for the non expert to refer to the expert.

This is followed in every field of knowledge in life. All human beings practically follow this fact in every matter. Religious day to day rules are not an exception. Non experts in religious studies need to follow the experts. We believe that every religious act must be based on real evidence. Evidence is known to the experts who spend long time in studying and researching the Islamic sources ( Quran, Hadeeth, Aql and Ejmaa'). People must refer in religious matters to the most learned Islamic scholar who is called al-'Alam.

From the time of the Prophet (SAWA) and the Ifallible Imams, Muslims who lived in places far away from them, were ordered to refer in religious matters to the scholar in their area. 
Taqleed is obligatory on non experts in religious studies (non Mujahid or non Muhtaat). With out Taqleed, non expert will practice religious acts with out authentic evidence and this is not accepted in Islam.

Wassalam.