Question 1, whether the "first gas propagation channel" is reasonable or not depends on the technical problems targeted by the scheme in its claim 1. If this description is enough to solve that technical problem, then it is reasonable. Or, you can give an example that "one gas transmission channel" can't solve that technical problem, and you can also determine that the patent of the other party is invalid accordingly.
Question 2, it advocates 1, 2, 3, 1+2, 1+3.
Question 3, then, depends on whether the other party's claim includes the scheme with only a 1 a2 in A, and if it does not include this scheme, it is also possible.
Question 4: If you are sure that D is not A (many times the other party will argue that your D is the A he described), then it is ok. (For reference only)