How Should TCP/IP be configured to allow an external application access to SQL via the same port #?
Not sure if I fully understand the question. But the internal applications should be fine about the port number unless there are also firewalls internally, in which case you just allow the port through there. 

For the external access, I'd consider a static port so you can stop doing what you have to do. In an ideal world you'd only really expose your SQL Server to the web server itself externally, so if the firewall can just trust the webservers IP only that is a good thing to do only, instead of just being wide open for that port for any incoming traffic. 

But you just ned to set an static port in the network config:



So instead of a 0 for dynamic ports, type a port number in for the TCP Port you wish to use.