Great post Nicole. We have this debate from time to time at my work. I'd opine that there's a continuum when it comes to work involved in going from ILT to elearning. At one extreme, you're spot on - "And since it’s already been instructionally designed, all you need to do is take the presenter’s PowerPoint files, import them into an e-learning authoring tool, and spit out a completed e-learning course—right?" This approach simply moves people from resenting the in-person experience to the online experience, and everything else in your post helps address this.
On the other side of the continuum is a well-design ILT course. Again, you hit the nail on the head when you say that you need to convert activities for the online world (ideally it goes beyond drag and drop or quizzing activities). This is where good ILT design can really, really help take time off the up front elearning design. I've found that when I have time for discussion in an ILT class, I can carry that over into an elearning module by having people type out their thoughts to a discussion question, and then keep that on the screen while actual comments from ILT participants also pop up on the screen for comparison purposes. Or where there's role play in the ILT, it can be converted into a branching scenario.
I suppose my point is that it *can* mean starting over (especially with poorly designed ILT materials), on the other hand, well-designed ILT really paves the way and makes the elearning design process much quicker (and more fun) and offers some insights (ILT participant comments for example) that you might not otherwise have if you had just been given an elearning project to create from scratch.