Hello

What is the purpose of Responsible part in Treatment section.

On the elements in general, the responsible has only administrative value.
Only on tickets where the responsible designates the person who actually works on the latter.

 to make Activity ID Sequential while creating activities

the id is indeed sequential. It applies to the creation on each of the elements and follows.
Now it's a collaborative tool, so if one of your collaborators creates an identical element at the same time as you, your sequence will indeed be disrupted.
You have the references to overcome that.

If a resource fills any timesheet,then its progress is getting reflected in Project status before even timesheet validation.This is an issue as after validation only progress should be applied in project.

I'm not sure I understand.
The working time entered by the resources is effective and real. It is therefore indicated immediately. We offer capacity planning and reality is a key element for monitoring.
Ask yourself that the carryover of this real on the project, only appear if a person has validated the work of these resources?