No I don't think they will do it offchain. They will keep it onchain to maintain seamless interoperability while they attack.
I can see you are confused. Unfortunately I don't think you understand how Bitcoin works well enough for me to explain this attack to you. Maybe just let the experts debate me. I will try to reply to you, but seems you are really confused.
Great argument.
How about you explain how a central off chain system can be vulnerable to double spends. Explain to me like I'm a child, I don't care.
Also, if they are not doing it off chain as you say, how on earth they are able to withhold a transaction I send from my non-cartel wallet.
I'm still waiting...