Could a student submit a create project after Unit 3? Technically, a design a scene project could have sufficient complexity to score well on the rubric. I think a student with prior programming experience could make the needed extensions to meet the requirements of abstraction and algorithms. The student may or may not be able to articulate answers to the written responses. On the other hand, I find that many of my students need the additional time in Unit 5 to develop a deeper understanding of both abstraction and algorithms. I point specifically to the lessons on variables, conditionals and loops. Although students are asked to use these concepts in Unit 3, the deeper understanding comes through additional application in Unit 5.
Are there situations where a beefed up Unit 3 would have to suffice? Yes (a 4x4 block schedule in 2nd semester might be one), but I would give all students the opportunity to access the content at a deep level.