1- We do Taqlid because we need to refer an expert in Islamic Law. Either you are an expert yourself, and you have the expertise to analyze thousands of verses and hadiths (remember the book of al-Kafi alone has over 16,000 hadiths), or if you are not an expert you must refer to one.
2- We follow one Marja’ because many scholars have stated it’s obligatory to follow the most learned. So if you believe one Marja’ to be the most learned (the best expert), you must follow him only. Yes, if you a believe a group of Maraje’ are equally knowledgeable, you can follow them. You don’t have to stick to one. Or if you believe one scholar is more knowledgeable in one field of law and another is more knowledgeable in another field, then you can follow them.
3- Over time, religious sciences developed and advanced. Those early scholars, even though they are not called Usulis, did use some Usuli principles to issue rulings. Over time, the science of usul became more advanced. Also, those early scholars had more clues to determine the authenticity of hadiths. They were much closer to the narrators and the Imams. In our era, due to the big historical gap between us and the Imams, we need more tools and Usuli principles to extract Islamic laws.
4- Yes that which is haram stays haram till the Day of Judgment. Scholars accept that. However, they might have different understandings of the conditions of a particular ruling. For example, if a scholar believes that a particular game was haram only because it was gambling game, then if one day it’s not a gambling game it becomes halal.
Of course most scholars have not accepted this understanding of chess. They have said we don’t know why chess is haram—maybe it’s because it was a gambling game and because of other reasons. Hence they have maintained it’s haram.