My view on this is that you have to focus your students on the differences between these future forms, not the similarities. If you use an example context where two different future forms are more or less equally imaginable, you haven't shown anything, and you're going to confuse your learners. This is why it's so important to prepare and select your example contexts so carefully. As far as exercises go, if the context is not clear enough to easily select one form over another, the question fails. It's your responsibility as a teacher to make sure you don't give your students poor questions.
As you say, this particular context was created by a professional writer or teacher as a special example of the use of the present continuous as a future form. (I made a mistake in my post #3 above, where I said it's 'be going to'.) Your job is to explain to your students why the speaker uses present continuous and not 'be going to' or 'will'. Of course, to do all this, you have to be very clear in your own mind about the differences between the forms. If you need help with that, let us know.
