Each embedded question has a "check" button, apart from the ones that are manually graded, it seems. At least it's that way with the essay question type.
It should therefore probably be prevented to embed manually graded question types until manually grading embedded questions is possible.
I think the tasks would be
- don't let one choose those with atto/tiny
- if any of those is embedded anyway, there should be an error message after filtering "Questiontype XXX cannot be embedded."