If there were some items of evidence that were much more important to the overall strength, then losing even just a few of these items might reduce the overall strength significantly. So the author must assume that there’s no items that are much more important than the others.

The author assumes the opposite — that the strength of a body of evidence is more than the sum of the individual strengths of items in that body. This is why the author thinks losing a few items won’t affect the overall strength.

The author’s conclusion concerns what happens if “a few” items of evidence are discredited. This doesn’t imply any belief about what happens is “many” items are discredited. “Many” means a lot — “a few” doesn’t necessarily encompass “many.”

The author does indicate that the two things compared are similar. Just as adding strands to a rope strengthens the rope, adding items of evidence strengthens the body of evidence.

(E) describes circular reasoning. The conclusion is not a restatement of any of the premises. The conclusion concerns what happens if a few items of evidence are discredited; none of the premises refer to this situation.