I must be missing something. This is a one year old water heater and all that's wrong with it is that one corroded dielectric nipple from a leak and they expect you to change the whole heater
I'm not there to look at it but if that's all it is, it's about a $3.00 nipple and a .78 cent copper coupling to fix it.
Is there a ball valve to shut the cold water off right there at the water heater?
No idea what "knob" is that needs replacing.