Rearranging the classroom setting together, making it more communication friendly is the first step for building atmosphere of community. Mixing them together might be dangerous. One of the solutions in this case could be arranging a group of three, but trying to mediate, to help them negotiate and avoid a conflict. Question number four is a very difficult one. The solution in each group and at each lesson would depend on the topic of conversation, on personal characteristics of the student, on group dynamics, on the proficiency level.
Fifth question offers to think over the discussion organisation.
Using L1 very often cannot be avoided altogether, at low levels the discussion is usually a mixture of L1 and L2. If a teacher tries to forbid L 1, it may break friendly atmosphere and make fairly strong language barrier even stronger. It seems better to encourage students to use more L2, helping them with some words or structures, but not criticising them directly. The next three questions are connected with a large area of methodology, i.e. mistakes correction. It might be a good idea to discuss this controlling function with them and understand what they would like.