"Is it the same thing when I say somebody is industrious versus somebody is diligent?" Yes. There might be a tiny shade of difference. To me, "diligent" carries a sense of duty, of doing all the work one is supposed to do.
In modern usage, "fidelity" is often used to mean faithful in romance, faithful in love, faithful to one's partner. The wider meaning is starting to be forgotten. If you use it in the wider sense, it will puzzle people.