[Rumil]

As A Brandybuck helpfully explains, Incanus seems to have originally been intended as a word from the language of the Haradrim, meaning 'North-Spy', then Tolkien changed his mind to make it a word meaning 'Mind-Ruler' or similar and derived from Quenya. I think a possible explanation suggests itself, which I'll expand upon and have a go at some mad speculation!

I think the name may be a pun from around the time of the Kin-Strife. It would have seemed a complimentary name derived from the scholarly Quenya, but in reality conceal an insult understandable to those who knew the language of Gondor's southern fiefs. No doubt Gandalf understood exactly what was being said. In the same way Orthanc meant both 'Mount Fang' and 'Cunning Mind' (or similar, can't find ref at mo) depending which language you were using.

During this time Castamir usurped the throne of Gondor's rightful king, Eldacar. After a great war between the two parties Castamir was defeated and killed at the battle of Erui TA 1447, but his sons and people eventually escaped to Umbar with the fleet. The party of the usurpers was strongest at Pelargir and south towards Umbar.

This raises the fascinating possibility that Gandalf involved himself directly in these momentous events, perhaps encouraging Eldacar and helping him build up support to retake his kingdom and even 'spying' on the usurpers. Scope for fanfic perhaps?