I wouldn’t cheat my key session by downgrading it to an easier workout. You can always build out your key workout with zone 2 the rest of the ride. I do this fairly routinely. You’ll take on some extra fatigue. But if you have the next day or two for easier rides, you’ll be fine
It might be but other than RLGL though there’s no detailed analysis of outdoors rides though so ticking either as complete will I think keep you on plan. RLGL will keep you on target if you overreach only. I think they have talked about the subject of extending outdoors rides on podcasts and recommended Z2.