Lesson

I think the short answer is no for Moodle out of the box.  Enrollment keys are for courses.   I am not the expert in this so there are probably addins and code work arounds.

In 1.6 there is lesson dependency setting under access control.   There are 3 criterion:  time spent, completed and % grade.   I believe completed related to the minimum number of questions setting in the lesson.   The current lesson is made to be dependent upon only 1 other lesson.  Thus for example, #2 would be dependent upon #1, #3 dependent upon #2, #4 dependent upon #3: but a lesson can only be dependent upon 1 other lesson.

There is an activity locking addin for 1.5.3  which is not part of the standard package.  It is similar to lesson dependency but deals with other types of activities.   My organization uses this in conjunction with Certificates.  Activity locking is my site administrator's responsibility in 1.5.3.  I am working in 1.6 and once the addin is ready for 1.6, then I will be playing with it  prior to it going live on our production server.

Hope this helps a little.