re Gardley, I don't know and there are multiple possible answers, at least some of which have been mentioned. With Garth, one of course is that something was promised and then not forthcoming.
In the theatre, as I think most know, the literary property is normally owned by the writers (who have a collaboration agreement among themselves), not by the producer (to whom the collaborators license their creative product), as it is in film. Garth never liked that and has tried to change it (e.g., in Ragtime) without success. The situation being described here seems like Garth is up to his old tricks. The bottom line is that credit (and content) are within the control of the authors and any changes would require their approval. Everything is negotiable, of course, but there is an extant contract so any negotiation would be within the context of that contract. And of course of course Garth is not one who abides by contracts. Perhaps the drama will be disclosed in what is pretty likely to be some lawsuits, another Garth feature.