Short No-Coverage Answer
The first part is confirmed: the Karistus are described as a 6D race based around Jupiter (232, 234). Jupiter is identified as "the abode of the Karistus race" and as the Cabal's "untouchable planet" — while the Cabal uses Saturn as its symbol, it won't touch Jupiter because it represents positive duality (232).
However, the second part — whether specific celebrities like Jared Leto are Karistus hybrids or agents — is never discussed. No celebrities, musicians, or public figures are identified as Karistus in the corpus. The Karistus do claim that "all Lyrian races are Karistus hybrids including humans and Taygetans" at a soul level (234), but this is a universal claim about all humans, not a statement about specific individuals operating as agents.
The concept of Karistus "descent" or hybrids working within human society is not elaborated beyond this general soul-level claim.
See also: What are the Karistus and what are they doing about Earth? (full Karistus profile)
Searched: All 834+ transcripts. "Jared Leto" and celebrity names return zero results. Karistus hybrid content limited to the general claim in 234.