For example: I have secured dice by a spell which can complete a full task on the adventure, but, it is an adventure with an order arrow, and the secured dice are needed for the last task. Now I roll my remaining die/dice and get exactly what I need to complete the first task, however, now I have used up all my dice. But I still have my secured dice, which would perfectly fit to complete the last task. Can I complete the last task using the secured dice, even though I can't roll any dice before moving on to the last task. Thank You!
Yes. In your example, technically speaking, you are really performing another round of rolling - rolling zero dice - and then resolving the final task. The die you have secured in your spell is a usable part of your dice pool, thus allowing you to meet the requirements of the adventure task even though you didn't physically roll any dice. Remember that the spell is discarded after you use the reserved die in that way.
However, if you still have more unfocused/unreserved dice to roll, you do have to roll them. In your example, you had exactly no dice left to roll after completing the first task and when moving on to the second. If you did have dice left, however, you'd have to roll them, which gives the risk of rolling a Terror result, which could have consequences. Thus it's important to note that, strictly speaking, you cannot complete multiple tasks at once (and in your example, you are not really doing so).