Hi. Can I just add some clarity to this.
You say your Cat5 runs from the attic. If it runs from the attic, do you already have a switch there or is this new cat5 just for the cameras? If temperature is your concern then look at an "industrial ethernet" switch which whilst more expensive, will offer the temperature range you need.
Your router being in the basement is fine however to connect the cameras to the internet you need to connect your attic camera switch to the router so simply 1 cat5 cable can do this as you have already found. Remember, depending on distance, you can extend these cables into the basement.
Port forwarding or NAT is exactly right, bear in mind when you set up the http port in the D14 admin menu it will have TWO boxes for http ports. When you choose your new port and put it in the box, put 80 in the second box. This will ensure the camera is still reachable over your LAN without needing to append the IP address with a port (port 80 is assumed to be the standard http port).
You can use a standard browser of choice to view the cameras with NO pluggins, etc however if using a laptop, download MX Control Centre or MX Easy for more functionality.
Shortly MOBOTIX are releasing a fully functional iOS App.
All this said there is nothing better than getting a professional in to configure the cameras exactly. A MOBOTIX CCTV system is a top end quality solution that needs knowledge to setup properly. You have made the initial investment by choosing one of the best systems available today, don't compromise that quality and you overall satisfaction by trying to save a few bucks.
Hope this helps