Unfortunately there isn't a way for project managers to allocate how much time per day a resource will spend on a task. Currently resource availability is set at the user level and then can vary by project over the duration of the project, but not by day. Typically we see that project managers know how much work it will take to complete a project and the resource will do what it takes to complete the work regardless of their work allocation. The system allows resources to be overallocated with planned work (shown in red) when the related panel is expanded and the appropriate fields are added.
In the reports module, you'll see 7 out of the box reports for resource load reporting i.e. Resource Availability, Resource Load, Resource Load per Task, Resource Overload, Resource OVerload per Task, Resource Underuse, and Resource Underuse per Task.
We do have an audit trail app in the apps marketplace, but it only captures changes to Start Date, Due Date, % Completed.
We have a feature request for a more detailed audit trail. The ID is ER-47.