[natechols]

7) Mitochondria (and chloroplasts) have double membranes, exactly like they would if they were smaller cells engulfed by the host cell.

8) There are multiple examples of ongoing endosymbiosis where the engulfed cell remains a true symbiont, not yet an organelle. Paramecium bursaria is my favorite - a ciliated protozoan with blue-green algae symbionts.

Bonus: there is evidence for secondary and tertiary endosymbiosis too.