Hi Guys, and thanks for the comments.
Dalecoy you're right...there are only 2 power sources; the supercap is dead. There isn't a battery in the battery compartment because it simply isn't necessary; its simply been moved from inside, supersized, then enclosed outside.
The Vue requires the 3V lithium cell only as a 3rd source when a functional supercap discharges completely, and this is where my external supply kicks in....I have a perpetually discharged supercap. The system is still powered by the solar panel within the ISS during daylight, and my external battery is charged during the day also (by the big 5W solar panel). At night, when the system attempts to draw power from the supercap and gets none, it switches over to the 3V (12V stepped down) battery, which feeds it happily through the night, and has more than enough capacity than the supercap could ever provide.
As a system test, I covered the ISS solar panel (the integrated little solar cell) to see if the external supply could provide adequately for its diurnal needs. It worked day after day perfectly fine, being supplied essentially from the 3V battery source only.
The number of power sources doesn't matter; the ISS just needs one good, clean, steady 24-7 supply and it will work fine. Rather quality than quantity!
Durability wise, the solar panel, buck converter and solar charge regulator are all solid-state, hardy components with good lifespan. The little lead-acid battery will need replacing in about 2-3 years, but its a cheap thing.
I mulled over the option of replacing the defective part within the ISS (the mainboard with the supercap encased in resin) but realised that I would be dealing with this problem again in the future, so decided on a more durable fix; that it works so well and at a fraction of the price is a bonus.
Hope this clarifies things a bit.