There are two flaws in the passage. First, R’s conviction with respect to the other investigators may be correct, for all we know. Second, there is the flaw that shows up in the answer choices- we cannot conclude on the basis of R being an asshole that R is incorrect. This second flaw (also known as the ad hominem fallacy, but you don’t need to know that) turns up quite often on the LSAT, and it is important to be able to recognize when this happens.
I think it’s good that you were suspicious of (A) due to the word jump, but in fact the word jump here does not exclude (A) from being the correct answer choice. R wrote a book on scientific research. The conclusion is that R’s book doesn’t merit attention from serious professionals. The attack on R’s character doesn’t mean that R’s book sucks- there are lots of ways in which they could have phrased this. The one that they chose- “not competent on matters of scientific substance”- is simply another way of making the point the attack on R’s character is irrelevant to whether or not R’s book on scientific research was any good.
(E), on the other hand, doesn’t capture either of the flaws in the passage. (Moreover, if the conclusion had been that R’s book is completely false, it would still be a bad argument)
I think the key to this passage is recognizing the common flaw taking place, which should set off red flags in terms of what to look for in the answer choices.