Hi,
"Chain" the lessons together one dependent upon the other. The
Certificate module should allow you to set a dependency upon any activity, including Lessons and Quizzes.
I don't like complicated dependency or conditional settings. While I have not done it, you can set dependencies in Certificate for multiple
activities. I would set the dependency on the last lesson in the chain. But that is just me.
Also, if you need to see some sort of record in Gradebook, why not give them one exit Multiple Choice question with an answer of "Continue" that jumps to end of lesson. Get clever with grades and scores, so they are basically immaterial to anything when it comes to Lessons but will allow you print a grade report showing progress of each student in the course.
Chris