Finally, I understood some benefit of estimation of tasks using hours and how to overcome my second question.
According to my understanding, breaking user stories into tasks is beneficial to understand “how much work the Team can commit to for a Sprint”, if we don’t know the team's velocity. However, before estimating tasks, the Scrum Master (SM) should prepare a team’s capacity planning to understand what is the maximum/total working hours the individual members will have in the Sprint.
After that, in Sprint Planning, the Team can break user stories into tasks and the SM can facilitate to monitor the individual task commitments against the total capacity of an individual person. If one member is exceeding the total capacity, the SM should inform the Team. In this way, we can avoid the overloading or underloading of a Team member within the Sprint.
Once all the Team members have committed to tasks with their full capacity, we can identify the velocity by calculating the committed user stories. However, if the Team is mature enough to estimate using user story points and we know the average velocity (after running 4+ sprints) then there is no need for breaking stories into tasks, unless the company uses individual commitments as their annual evaluation Key Performance Indicators.
To avoid exceeding the Sprint length, the SM should prepare a capacity planning before individual task commitment. The SM should monitoring the capacity of each individual Team member by examining to his/her total capacity so that the Team will not exceed the Sprint length with the estimation. However, in this method, we cannot pull user stories to Sprint Backlog by considering velocity, then break them into tasks and again estimate using hours. Either we have to pull stories by considering the velocity or we have to pull stories using the Team’s capacity.