I dont think I missed an explanation in the anime, but I'm not sure if I missed on in the manga. How is Zamasu supposed to be alive in Dragon Ball Heroes? Is the same Zamasu?
The Dragon Ball Heroes anime is simply promotional material and you shouldn't really dwell on certain factors concerning the plot such as this because you aren't going to get any precise explanation. However, the main series has shown that Zeno 's actions can be reversed by using the Super Dragon Balls. So maybe Zamasu was revived by Fuu as a part of his Prison Planet experiment or by someone else. Although, this is just speculation.
As for your other question. Yes, it is the same Zamasu who has fought Goku and co before. He clearly seemed to recognize Goku when he arrives alongside the Grand Priest.