it's an interesting situation all round
on the one hand, we have two out-of-place alts for joker that don't really seem to fit into anything--his purple thief skin and beige uniform--which may have been put together quickly to fill two gaps for a scrapped kasumi alt in the last two costumes (as the datamined files suggested)
on the other hand, kasumi being intended to fill a basic skin/alternate costume slot like bowser jr. or hero makes no sense for several reasons, among many others:
- royal is 6 whole months out from joker's intended smash release date
- she has completely different weapons
- she has different animations for movement, a different all-out attack, and likely different taunts with a different personality expressed
- the model was basically finished and fully-animated before going missing
- she uses bless attacks, not curse (like eiha/eigaon)
coupled with joker using stuff from P5R like the grapple attack/pull confirming sakurai's full knowledge of the royal, it seems much more likely to me that her status as an 'alternate costume' was just them developing her model using joker as a base (like the smash 4 clone trio) before splitting her off and sorting out the differences
i'm also not sure i understand the notion that kasumi is less likely the closer we get to P5R - she already has an english voice and royal's western marketing is kicking off months in advance, which is incredibly unusual for atlus, and indicates that they're taking a much more active approach to its promotion. if she's in, kasumi was most likely already near complete around the time of joker's release - i don't think it's a matter of including her 'on time' so much as it is them waiting to drop her at the most promotionally-sensible time (probably around the time of 6.0.0/the royal's japanese release in november alongside whoever challenger pack 4 is).