As a medic it is certainly not illegal to stop CPR. In the first instance, if you are physically incapable of going further, (CPR is exhausting) then you may stop if there is no other assistance. I could imagine a scenario on an aircraft where there is no possibility of essential support such as intubation and respiratory support, that the CPR is evidently futile.
I have, as first responder, (not in an aircraft, it was in a pub) refused to get involved due to the threat of violence.
I have, once, responded to a CC call for assistance. Fortunately the person had suffered something like a transient ischeamic attack (mini stroke) but maintained a full pulse and respiratory effort. CC were very good, although asking questions about diversion that I had little answer to. I did not know how to answer "do we need to divert?" All I could do was to describe the condition of the person. Fortunately I did not need to intervene with actions such as CPR but the patient was semi-conscious and clearly unwell. I had little idea of what a diversion would mean in time or effort. (This flight at the time as over Europe from Moscow) This question was asked very soon after I attended to the patient so I could not judge any improvement or deterioration (Improvement was evident a little later) My reply was that I did not know.
I struggled with the issue of diversion. it is far from our minds about where the aircraft should go. Possibly for CC or CRM meetings, it may be useful to have medics attend to describe what questions need to be asked of them. Possibly a direct question - "Do we need to land ASAP? And if so, why?" The second question is not impertinent, it will focus the first responder's mind on urgency. If I had been asked that question directly in this case I would have answered no, and would have said that the patient has a pulse, is breathing and is responding to painful stimulus so is not in extremis, so immediate assistance is not required. Treatment within one to two hours would be optimal. That leaves the professionals on the aircraft to decide what to do.
Maybe for CC it is asking the correct question of the first responder.