Vibhishan got immortality as per Ramayana too as discussed in the following post:
But according to Uttarakhanda of Ramayana, Ravana also got a boon from Lord Brahma and
he was incapable of being slain by Nagas, Yakshas, Daityas, Danavas and the Devas.
This not required that an immortal have to be more powerful than others. For instance, there were many powerful warriors than Kripacharya in Mahabharata.
The very first thing was to snatch Lanka from Kubera. And as you mentioned in your question that "his soul inclines not to unrighteousness" so how could he snatch Lanka from his elder brother.
How he could be king when his elder brother Ravana was there? This is not strong point though but still can be considered as valid.