Как-то мне future simple здесь не кажется к месту(

Это предложение в вопросе из учебника👍 Обожаю

Я тоже)))

WILL is very correct here. When we speak of plans, we use GOING TO. When we speak of a strategy for an undertaking, and especially one that involves several steps/a process, we use WILL: A: I want to go to Amy's party on Saturday, but I'm still grounded for skipping school last week. So I'm going to take my father's car. (= that's my plan). B: How the hell are you going to do that. Your father's still livid with you. A: Easy. I'll fix him a martini after dinner and put on a funny movie. Halfway into the movie, I'll make him another one. He'll go to bed around 9 and sleep like a baby. When I know he's asleep, I'll take his car keys out of his jacket. I'll text you and you can run over. We'll roll the car down the street and start it on the corner and it's off to the party)))

Epic! Really, really interesting. Thank you very much. Значит ли это, что вопрос What opening remarks are you going to use? в задании из учебника был бы неправильным?

No, one is referring to what he PLANS to do, and one assumes that he has a PROGRAM planned and wonders what he plans to do by way of opening remarks. So they’re asking more or less the same thing, but from different perspectives.