Please tell me if I understand this correctly (PIM-SM)

Obligatory not a network professional, but am tasked with maintaining a few networks as part of my job. We are making some changes to our internet connection and am trying to understand how a few things work.

First off, there are 3 networks involved here.

The first (N1) is a network of machines which process data from hardware "data collectors" if you will. These collectors are cameras and other various sensors. This data is then transmitted through the use of this network's servers and internet connection (4-T1 lines) to a monitoring center which processes the data. This center also monitors all of the devices on this network remotely.

The second (N2) is a network of with it's own router, multiple hardened switches and many cameras. This network is managed by a desktop with a wired connection. This network currently has to active internet connection and is split into several VLANS.

Network 3 (N3) is a government network which receives the multicast traffic from N2 and then hosts it on a public website. N3 receives this traffic through a port on the router of N2, set up in PIM-Sparse Mode.

Currently, we are upgrading the internet connection of N1 to a fiber connection, and wish to share this internet with N2, for the purposes of remotely managing and monitoring the cameras on that network.

Am I understanding what PIM-SM is correctly? I believe that it means that N3 is setup to receive the traffic from N2, but cannot interact with N2 in any other fashion. Transversely, N2 has no other access to N3. Does that sound correct? So does the PIM-SM create a virtual barrier which only allows one way traffic, being multicast data to a preallocated point, the RP?

Bonus Question: What is the most efficient way to share N1's internet connection with N2 while ensuring the two networks remain separate on their own. Both N1 and N3 have a router and a variety of switches.

If any clarification or additional information is needed, let me know.

Thanks in advance.

Edit: Would having multiple static IPs from the ISP achieve sharing the connection to two networks? To do this, I'm guessing there is a modem type device which splits the connection correct?