I usually use routers as hosts unless I need something that a router can’t do, such as iPerf.
When you add no ip routing on a router, you disable routing and it will need a static route or default gateway when you want to reach destinations outside of its own subnet. I like to use routers as hosts because it’s simple and they are easy to add to a topology. No need to understand Windows or Linux.
Also, when you see a simple line that connects those devices, it’s a L2 segment. In reality it’s a switch but it doesn’t require any configuration. Even in an emulator, it could be an unmanaged switch. I usually do this not to clutter the topology.
That’s not a silly question! It will help you and others to understand the way multicast works.
Now, multicast is a service typically initiated and managed by device applications. For example, if you have a multicast service on your enterprise network, say streaming video or audio music being shared among multiple devices, your PC, for example, will connect to the server using unicast communication. During that communication, the application on the PC will negotiate with the application on the server, using whatever software mechanisms have been programmed into them, to allow the PC to join the required multicast groups. The server will inform the PC about the multicast groups it needs to join, and it, in turn, will send out the IGMP join messages needed. Now, details of how this works depend highly on the server and client applications running on the PC and the server.
So, multicast in a production environment is primarily a mechanism initiated and managed by the applications themselves. They just require that the underlying network infrastructure is able to support the multicast operation using PIM, IGMP, and the related protocols and network features. Does that make sense?