This is really five different questions compacted. Let me take a few at a time.
DO NOT UNDER ANY CIRCUMSTANCE start IIS. IIS will conflict with Apache for port 80.
Apache is your web server and it will serve up the EMR.
If you can access http://localhost/openemr, then you can point your users to http://youserveripaddress/openemr.
So go to a user pc on the same network and open the browser and type in the http://179.20.128.69/openemr.
Replace the 179.20.128.69 with your network IP address of your server.
If you are planning on having clients access the server across the internet from remote locations will take a few more steps.
to do what you are proposing you are going to need something like
http://www.symmetricds.org/doc/3.8/html/tutorials.html#_demo
May I suggest a simple answer to failover? If you want to make sure that there is a central data repository and that if the internet service to clients is compromised. Why not have more than one internet provider in a location. In some of the work I have done, I have seen up to three internet service providers in a facility. They are for redundancy and consistant connection to the outside. There should never be a time that all three are down. Placing the modems on UPS gives life to the network internet connection for upto two hours in case of a power outtage.