Does final nirvana conventionally exist - especially in the lotus sutra? Does every tradition, especially ones descended from the lotus sutra and the Buddha's immeasurable life, claim that parinirvana is a conventional existent?

Assuming that it's not a dharma, I don't know.
In my experience, zen and tibetan buddhism do not claim that the self of an enlightened one can be completely non existent. In fact a zen master when asked this question might answer "Uncertain." or "yes" or "no". Ask for definite answers is a limiting factor in an unlimited universe.

The following article discusses your question in far more detail than I can:

http://www.iep.utm.edu/pudgalav/


  Here is an excerpt that applies to your question: But there is no
  doubt that they affirmed the reality of the self or person, and that
  with scriptural authority they held that the self of an enlightened
  one cannot be described as non-existent after death, in “complete
  Nirvana” (Parinirvana), even though the five “aggregates” which are
  the basis of its identity have then passed away without any
  possibility of recurrence in a further life. These five are material
  form, feeling, ideation, mental forces, and consciousness.
  
  It seems, then, that they thought of some aspect or dimension of the
  self as transcending the aggregates and may have identified that
  aspect with Nirvana, which like most early Buddhists they regarded as
  an eternal reality. In its involvement with the aggregates through
  successive lives, the self could be seen as characterized by incessant
  change; but in its eternal aspect, it could be seen as having an
  identity that remains constant through all its lives until it fulfils
  itself in the impersonal happiness of Parinirvana. Although their
  account of the self seemed unorthodox and irrational to their Buddhist
  opponents, the Pudgalavādins evidently believed that only such an
  account could do justice to the Buddha’s moral teaching, to the
  accepted facts of karma, rebirth and liberation, and to our actual
  experience of selves and persons.