Hmm I think it's a system of theirs? To add new members that way to make it more interesting. Though your claim is also possible.
However, I think Marianne is truly missing or else the crown prince and moon owl wouldn't look too worried. And they wouldn't have made fl to pretend to be 'real marianne' since moon owl already mentioned it before that to make it more mysterious they would've made marianne practice the things she can't do in order to make it harder for the other jewels and viewers to figure out who marianne truly is. And they already had a 'fake marianne' anyway. I think when they found marianne then she will be added as a jewel there then our fl's role will be accomplished.
Only Pearl's identity hasn't been revealed. Does this mean she is Marianne and the she-is-missing scenario is one of the twists?
Or there's really no Marianne among them?