RLA? Did you mean McCaskill-Bond? Assuming you did, the key is the "s" when it say it applies to employee groups. So it isn't prohibited.You're confused about several things. First, the NMB will rule and USAPA will no longer exist, possibly as soon as April.
Second, when the APA becomes the CBA for all of us they will be bound by a Duty to Fairly Represent all of us. That's why they will appoint committees to represent all sides for the arbitration. And that's also why they will allow for separate West representation (and they will ignore the East's objections). Since you claim to have read the court filings surely you know that any deviation from the Nicolau list will very likely elicit an injunction from the inevitable West lawsuit.
Our reps feel a Protocol Agreement won't be reached and the NMB ruling will make it moot. Let's see how things transpire this week.