Who is Devil Z? I know he is the emperor of the Destrons. But who is he in relation to the rest of the G1 Transformers Universe (eg. Megatron, Primus, Dark Nova, Unicron, etc.)?
The show never properly goes into Devil Z's backstory, so it's never really explained. A video that was released after the series ended tried to explain his origin, saying that he's a super-energy being from the far reaches of the universe, but that was still pretty vague.
It wasn't until 2018 that a true origin for him was finally made: Back when the Quintessons ruled Cybertron and enslaved the Cybertronians millions of years ago, the Quintessons were in possession of the Matrix. Since the Matrix takes the wisdom and experiences of its wielder and turns them into a powerful energy, the wisdom and experiences it took from the Quintessons created an evil energy within the Matrix. When the Cybertronians revolted against the Quintessons and kicked them off Cybertron, they took the Matrix back from the Quintessons and dumped out the evil energy that had been built up within it. This evil energy was cast out to the far reaches of the universe. However, the evil energy persisted, and over the millennia it evolved into a lifeform, creating Devil Z. He swore vengeance upon the universe for being discarded, stole the Godmaster Transtectors from the factory that built them, and hid them deep within the Earth so that they could absorb the power of two other energies hidden within the planet: Zodiac (the energy of the cosmos) and Angolmois Energy (the energy of the Earth itself). Devil Z also absorbed these two powers, with his true name revealed to be "Devil Zodiac".
But, again, almost all of that is never actually given in the Masterforce cartoon itself.
Why is Japanese G1 calling the Autobots Cybertrons and the Decepticons Destrons? Aren’t they the same things?
That's just their Japanese names.