After long and seemingly fruitless analysis of how /j/;/t/ variants (such as [-junga]/[-tunga]) relate to /v/;/p/ variants ([-vunga]/[-punga]), I think I've finally made headway. I always thought the /v/ variants were some kind of formality markers and/or interrogative/reporting progressive-indicative mood. But it occurred to me just now that one uses these /j/ and /v/ variants in actual relation to each other:
angirrannut isilauqtunga anigamali tavva takuvagit
"I first entered my home then, when I came out, I saw you"
-the grammatical sequencing of /j/ (isilauqtunga) with /v/ (takuvagit) in the above construct shows an interdependence between /j/ and /v/: it is used in a telling of a story, but the indicative /j/ is adjoined with a /v/ adjunct phrase (which functions as a past-progressive mood).
From a morphosyntactic perspective, /v/ grammatically completes the /j/ by adding to the past tense,[-lauq-], its "progressive" quality.
The /v/ variant is also used in formal address to the second-person:
"I will tell you once I've found something out"
-but here it is also completed with a sort of a /j/ variant ([-lunga]), which technically may be regarded as a /j/ variant for the sake of the dichotomic argument: /j/ vs /v/.
Methinks this /v/ variation is a progressive marker that is always complementary to /j/ indicative. /j/ is the main verb while the /v/ a secondary verb tense thereof.