Dear PathFinder007,
I'm happy to respond. Here's what I'll say. Some of the GMAT SC practice questions floating around on GC are very good. Magoosh and MGMAT have very good questions, and I have been impressed with many Veritas questions as well. Many others, though, are atrocious. Think about it. Not every car company that says, "We have the very best car on the market" actually has the best car---the claims of advertising often overstep the truth. Well, when a GMAT Prep Company says, "We have high quality SC practice questions," that's also an advertising claim, and you shouldn't just naively take them at their word. Many SC questions I have seen on this site are truly horrible questions. Caveat emptor!

Let's look at this questions. Here's the text again:
In its most recent approach, the comet Crommelin passed the Earth at about the same distance and in about the same position, some 25 degrees above the horizon, that Halley’s comet will pass the next time it appears.
(A) that Halley’s comet will pass
(B) that Halley’s comet is to be passing
(C) as Halley’s comet
(D) as will Halley’s comet
(E) as Halley’s comet will do
Choices (A) & (B) & (C) are abysmally wrong. Choices (D) & (E) are both perfectly correct. There is nothing at all wrong with (E). This question has two correct answers, and three other choices that really wouldn't tempt many test takers. This is an abominably bad question. I would give this question a grade of an F.

My friend, you are perfectly correct ---(E) works just as well as (D). The problem is not in your understanding. The problem lies in the question itself. Doing low quality SC practice questions will only confuse you---it will not build the skills you need to be successful on the GMAT. Here's a high quality practice question:
https://gmat.magoosh.com/questions/3582

Does all this make sense?
Mike