I wonder how far ahead they had all of this mapped out? For example, the Red Skull bit. When he disappeared at the end of the first Captain America, did they already know what they were going to do with him? If so, that is mighty impressive.
When he appeared, I did a literal fist pump in the theater. Wasn't expecting him at all. As far as the plan, I don't think they knew THAT specifically, that he'd be guarding the Soul Stone 10 years later, but they knew (as most of us suspected) that he wasn't dead and could be used in the future, just removed from the board temporarily. Having him basically act as a stand-in for Death was just a stroke of genius by the writers.