To answer your questions simply: yes to all.
You can put multiple DVD quality movies on a Blu-ray disc as well as regular data. However, the device you are trying to access would need at least a Blu-Ray ROM drive. So if you do put a Blu-Ray disc with regular DVD content in a DVD player, it would not read. But if you put it into a Blu-Ray player, there should be no problem.
To answer your questions simply: yes to all.
You can put multiple DVD quality movies on a Blu-ray disc as well as regular data. However, the device you are trying to access would need at least a Blu-Ray ROM drive. So if you do put a Blu-Ray disc with regular DVD content in a DVD player, it would not read. But if you put it into a Blu-Ray player, there should be no problem.