What is important is the intention to fly the airlines ETOPS speed. What you actually fly during the diversion is up to the PinC on the day. You can expect to be questioned by the authority why you deviated from your airline published ETOPS approval.
By briefing anything different your instructor is correct that you are not meeting the ETOPS certification requirements held by your airline. You can however brief different failure case options but the standard ETOPS speed must be the primary option you intend to use, i.e. that is the approval your airline operates under.
I have had this discussion with two different CAA airline inspectors in the passed year from their observation check flights.