1st issue is to get into the router and bypass the password in order to do this you will have to take down the router and enter into bootstrap mode. This will cause an outage but at least you will be able to create a new log in.
Password recovery method: [video] - Password Recovery in AOS
As far as Ethernet vs Fiber. I always choose Fiber it has been tested tried and true but if it is not an option the Netvanta switch does have an option to test Ethernet Cable to assist in troubleshooting.
Troubleshooting Ethernet with a Netvanta Switch: Cable Diagnostics Troubleshooting Guide
Concerning replacing Ethernet Cable with fiber optic cable: Given the right conditions, copper Ethernet cable is good for speeds all the way to 1000Mbs. For your purposes, 100Mb (100BaseTx) is adequate. Your internet connection is a 50Mb connection, so going faster than 50Mb from your switch to the Netvanta 3430 is of no real benefit. You'd just be sending the packets to the Netvanta 3430 at a faster rate, but they would still be buffered whle getting onto the 50Mb link to T/W. I don't think you'd see any benefit from adding SFPs and fiber optic jumpers.