Yes I do think assigning the time frame for each task is a tough ask and I think to assign the task the appropriate time the PM has to look into the past to see if such a task was performed before and what was the time required for that task and does it match the current task at hand. The manager has to do his calculations and come up with the ideal time for a particular task. Also, I think an experienced PM would know how much time a certain task would require and will assign the time perfectly.
For the second question, I think if we finish a process early we can utilize the time for the other phases.