1) In bigger cities first year prescribed stipend is 1000 pm, 2nd year is 1250 and 3rd year is 1500/-. But usually firms will pay more than this in cities/metros like one can expect anywhere in range of 4-6k pm in big cities. In Big4's you can expect 2-3k more.
2) Regarding leave, i think it is 6months in total articles period one can avail.
3) If a student can be able to join in October 1st week itself then you can be eligible to write in May 2014 because by that time you will be fulfilling 2.5 years gap for giving final exams, as the rule says one can give final exam in last 6months of articleship.
4) Regarding joining, in my view, you can join mid-firm if it is a reputed one instead of waiting for big4, because you gets lot of exposure in multi-areas, which is lacking in Big4. However, doing articles in Big4 adds value to the resume in future. One should be ready to work more time and to go outstation audits. As per current transfer rules, for the first year of articleship there is no restriction for transfer. However, I am unaware whether such transfer also needs signature of the Principal under whom you may work, if you joins mid firm. In case signature of the Principal is required even for first year of transfer, then it depends on the Principal whether he do signature or not. Think before itself and take a good decision. If you are sure that you gets joining report from E&Y, then join there itself if you want to work in only one/two areas max. This will holds good only if you dont want to start a Practice after your course. All the best.