Multiple Zamas is perfectly coherent with the story. As we saw when Vegeta launched the omega blast, after resisting the defusion and fusing at a celular level every piece of Zamasu became able to become a full Zamasu due to their inmortality.
That army of Zamasus the result of Goku's Hakai technique, and all those pulverized cells turning into a full Zamasu (that's why in the end of the chapter Goku asks Beerus if they could've had deal with Zamasu, and Whis replies "we have something that's much better than your jar".
Monster Zamas it's only coherent in that it escalates the asspulls of that saga to the very maximum. If Black's Magic Zenkay, Trunks SSJ Rage, Trunks' Mafuba, the fucked up power scale that was inconsistent not just between sagas or chapters, but even between scenes of a chapter and the Genki Sword hadn't been enough for anyone to understand that Toei's writters don't give a fuck, Monster Zamas was there for the sole purpose to make everyone realise it.
Now if you die you fuse with the universe because reasons. Well, to give them credit, if Toriyama wrote that Zeno-sama erased the whole parallel world then I think the writters at Toei gave a reason for him to do so.
They took Zeno for your usual "good kiddy character" and that's why the manga is so much better. Why would the gods of destruction be so afraid of anime Zeno sama?
In the manga they show you what can happen if you mess with Zeno. Zeno is not a kid you can easily fool to do exactly as you want, because he is too monstruously strong and really doesn't give a shit about blowing every single universe (as it's later shown in the universal tournament saga when they wanted to erase 8 universes just because they became bored of their game).
