I've always found it strange how WS-19 could be so expedited compared to WS-15 and doubted that J-35A uses WS-19 from the get go. Sure it benefits immensely from WS-15 experience and technology being directly available to the WS-19, cutting down the development cycle time and cost.
WS-21 being used for introduction block J-35 and J-35A makes sense. It is an engine below the tech level of WS-19 and is basically equivalent in tech to WS-10B/C/X.
WS-19 should reach maturity readiness after WS-15 and WS-15 has barely reached maturity readiness for mass production and application yet.
The confusion with J-35A already using WS-19 probably stems from the alleged news that J-35A has been tested with WS-19. That's not the same as maturity to apply on all production fighters.
WS-10A reached maturity readiness in the early 2010s for widespread adoption on J-11B and onwards land based Sino-Flankers. WS-10B only reached maturity for J-10C application in the mid to late 2010s.
We should expect WS-19 to be on later block J-35s.
On the question of carrier J-35, WS-21 has had years and years of head start vs WS-19 for navalised version and saline environment modification, testing and evaluation over those years.