I would point first, as it's the less interesting, that "pheobus" is a pedantism. Either Fèbus or Febo could work, as they were a contemporary use.
Then, there's a mistake. Navarre didn't had, except Lower Navarre proper, territories north of Pyrénées. It's admittedly a bit complicated but basically there was
- Kingdom of Navarre
- Viscounty of Bearn, technically independent from Navarre and France
- County of Fois/Foix, part of France
It's why the Kings of Navarre, including after Castillan conquest of Higher part managed to be still part of high aristocracy. So, these territories were pretty much safe anyway.
That said, I don't think it would have been enough to push back Castillans if they have tried as OTL to takeover. Navarre became a buffer state between Castille, Aragon and France and when the two former sides agreed that it wasn't necessary enough, well...it was done.
Assuming Navarre would be able to intervene during the war (I doubt it, OTL they eventually joined Isabella's side by defiance to France support of Joanna), I don't think they could have influenced that much on the result : Papal annulation of Joanna's union with Portugal, very few foreign support safe nominal for Joanna, and huge support of cities and middle and low nobility for Isabella...
That something Navarrese support couldn't easily breach.
Now, it would have interesting consequences on House of Foix, possibly their link to Bourbon dynasty.