I don't think I missed an explanation in the anime, but I'm not sure if I missed one in the manga. How is Zamasu supposed to be alive in Dragon Ball Heroes? Is it 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 arrived alongside the Grand Priest.
At the end of Dragon Ball Super, Android 17 wishes for all of the erased universes to be revived. And remembering how Zamasu was transformed into Infinite Zamasu, he counted as a universe as well. With Android 17's wish, Zamasu was revived as well. Although, it is a mystery as to how Zamasu got his normal fused body if his (Future Trunks') universe was revived.