the un-shot series finale

I can't guess how they would have ended the series even had they been given the chance. Their budget was not huge and TV sfx at the time were not overly sophisticated. Also any solution to the alien problem would - at least in order to fit in appropriately with the series' mood - be somewhat dark, hard won - at a very high price.

I doubt therefore that they would have gone in for an interplanetary war scenario with an Earth vs the Flying Saucers-type war.
Maybe - a la H.G. Wells - they could have infected the Invaders with a very tough, "innovative" earth virus, created or at least developed in a lab, and disseminated to the aliens via their regeneration tubes or in other ways.

This could have ended the series with a moral confrontation between Vincent and a dying alien who knows that his entire race is now doomed - doomed largely to Vincent's own "war efforts". An ending like that would have been dramatic, inexpensive, and fitting... well within the series' budget, sfx, and mood.