I doubt they will accept compromises on matters of law and if they did, it would only be with the acceptance of both parties? I am not sure the club would accept that if they believe in their position (it could be they have a wider strategy than just Etihad). And even then, it would allow another party, say Villa, to challenge the lawfulness of the rules and the changes.
My uneducated opinion is that the tribunal has to make a judgment purely on the legal arguments put by the PL and the club. But I am happy to admit there is a very good chance I have no idea what I am talking about :)