It states that mission tasks must be dealt with from left to right.

1) Does each task have to be completed before moving on

or

2) If I fail a task do I proceed to the next task.

A related question:

If I fail the first task, but still have other soldiers available to allocate, can I assign another soldier(s) to that task (provided the symbols match).

Finally, I think I have read this, but can't remember where.

1) For non-enemy mission tasks, if I fail it I think the soldier does not die (is that correct?).

2) If that task is failed, can I attempt the task again? If so, does a different soldier need to attempt it?

Each mission task must be completed, left-to-right, before the next may be attempted.

Yes, if you fail and lose the assigned soldiers but have other soldiers with matching symbols who weren't assigned on the first try, you can try again with the other soldiers.

Incorrect, the soldiers are still lost on a failed roll.

1) For non-enemy mission tasks, if I fail it I think the soldier does not die (is that correct?).

2) If that task is failed, can I attempt the task again? If so, does a different soldier need to attempt it?

1) For non-enemy mission tasks, if I fail it I think the soldier does not die (is that correct?).

2) If that task is failed, can I attempt the task again? If so, does a different soldier need to attempt it?

1) If you roll a Loss on any Task, you lose (or Exhaust) the Unit(s) involved. The only way the Soldier wouldn't die is if you had UFO Flight or Revive and an appropriate Unit on hand to use them with

2) After rolling a Loss, you may continue with the same Task so long as you still have eligible Units available - either due to saving an Interceptor with Defense Matrix or because you're in ground combat and still have some assigned Soldiers left (either due to Revive or because you didn't commit all the assigned Soldiers to that roll).