I believe they could, either Jade or Mykayla, as they went thru qualification. I am not sure but I believe that BEFORE qualification, they could sub one of the alternates (Kayla or Emma, as Kara and Leanne were out because of Covid).
When we discussed the 'alternate process' right after the trials, it always sounded like Riley never had a chance, as taking one-event specialist would be risky if the +1 gymnast was there for the alternate spot on main team.
But then I read somewhere (maybe Simone's instagram stories? Not sure) that AFTER qualification, they would have to use one of the +2 because they went through qualification, while the alternates didn't. So it sounded like BEFORE quals, they could use any of the alternates, while AFTER quals, it could be only Jade or Myk (I believe they have to submit the rosters 24 hours prior so that would have been the deadline for the subs).
Maybe I am wrong, but that's how I understood it. I tried to find some kind of official document (FIG or IOC) that would explain the process, but no luck.