All that said, I think the author is :
- Overestimating the distribution ability of coins by one casino ... and that is even compounded by the fact that , IIRC, in this manga and LN , there is not a world governement and coin. But even if there was, getting enough of distribution of coins among the populace is a thing that would take time, most likely months or even years. That leads to ...
- Glossing over the fact that would discover the explosives inside the coins quite fast. The coins would not weight the same and , more important , would not sound the same. Most likely they would not be usable in machines ( due to the weight diference ). Someone would bend or cut one of them , even if by accident. In other words, you would have a very limited time window to distribute the coins before someone noticed them, but you are doing it from only a point of entry . After that you would still need to not be ...
- Forgetting to ensure the coins could make real damage. Remember, coins are small and you would still need a lot of it to be metal to make sure they would not deform under human touch. So, you can't put much of explosives inside one. More, you can't make a directional charge without it being noticeable from the surface of the coin, so the explosion would not be directed ( like a bullet is ). So, a coin with explosves would probably not be worse that a exploding cell phone battery ( and most likely even less ), and while they can be somewat damaging, they are definitely not in the magnitude the manga is portraing the authors of the plan imagination. And even this was not a issue ( say ,enough coins together could really make a fuss ), you would need our masterminds ...
- Finding a way to activate them all at the same time ( that's their plan atleast ). Coins, again , are small and that ( science incoming, brace yourselves ) limits the radio wavelenghts you could pick from a radio inside it hard ( and I'm even assuming that they could put a radio and detonator inside each coin without loss of explosive payload space
). A coin sized antenna would be useless to pick wavelenghts that can be emmited from space and would only be useful to pick the higher UHF spectrum at best ... most likely it would be only really useful at microwaves, that have relatively small radius of usage inside our atmosphere. So, unless they hijacked all cell phone and TV antennas, that would be hard without a very convenient ability user that was not shown. And I can think of more devious and effective damage you can do with that kind of hardware under control ...
And to this you would still need to add that people would need to panic and that the state would not be able to make a coin recollection fast enough. As someone that lived during one of the biggest coin recollection efforts around ( the adoption of the Euro ), I find that to be atleast disputable. All in all , IMHO it is not a very well thought plan :/