A.
would begin as scheduled if the member nations
will settle the dispute over each of the nations’
issue: verb tenses. "would" will not go with "will".
B.
were to begin as scheduled if the member nations
will settle the dispute about the nations’
issue: same verb tense issue as A.
C.
would begin as scheduled if the member nations
were to settle the dispute over each nation’s
correct: "would" rightly goes with "were". Correct meaning implied - if members settled the dispute -> implementation would begin as scheduled.
D. will begin as scheduled as the member nations settle the dispute about each nation’s
issue: meaning issue. The sentence needs to capture the essence of a condition.
E. would begin as scheduled when the member nations were settling the dispute over each nation’s
issue: meaning issue. simultaneous action is implied, rather a conditional meaning is required.
Correct me if I'm wrong in any answer choice.