I've just added to my work-in-progress course format the option to set a current interval per user. So, if the interval is set to weekly and per user it will set the current topic by weeks from the user's enrolment time (other possible intervals are daily, monthly and custom). There is also an option to force the current topic such that the user will always see on the course site only the current topic.
But again, while this simplifies the user's view it does not yet prevent access to activities in non-current topics by direct urls. This is because access is conditioned by context and by (arguably shortsighted) design there is no context for course sections even though activities are essentially parented by the course sections.