|1) When all the bandwidth is used up to the point where nobody has decent connectivity anymore.|
2) The simple response is, yes, every user affects every other user. Think about it this way. You have a single pipe coming in that you are splitting up. At some point all the separate connections aggregate and go back into the single pipe. Therefore, every user affects every other user regardless of which network they're on.
3) This is a little tougher to give a definitive answer to. I suspect your 4 static IP's are all within the same subnet and are therefore not really separate. But from the sounds of it, internally, you have them separated.
I suspect the network connected to the SOHO Router has the IP as provided by your ISP configured on the WAN (external/internal) port and you are probably using the router's built in DHCP router to provide TCP/IP settings to clients connecting to it. If this is true, clients on that network have no direct route locall to the other network. But, they may possibly have a route through your ISP's network.
You may be able to test from the management interface of the netgear router. It should have a feature to allow you to ping the other static IP you're using. If you can, and do the ping test, and the static IP from your accounting LAN replies, then you have communication between the two. Alternatively, you could call your ISP and ask them about this.
On a side note, I hope you're using a SOHO router on the account network as well. One can't be too careful, especially with sensitive information you don't want outsiders accessing.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.