Useful for a lesson on 'drop in' clauses.
The comma splice in example 4 needs tweaking: 'This is the main sentence, it makes sense on its own.'
This clearly highlights the problem of commas : they are annoyingly tricky, even for adults!
It would have been nice for you to message me before writing this. It is people like you that make others refuse to share resources and ideas. As for your use of the term "drop in" I do hope you are giving it its tehcinal name when you teach!